Ric, I've gone through what is available on the Ameliapedia on FN and I can't see "redemption" as a motivation for FN I'm afraid, nor does FN come across as "desperate and rushed" to me.
The sequence of events in FNs divorce and remarriage as far as we know them seems fit with him only deciding to divorce this first wife after he'd met someone else. In these cases people tend to be excited about starting something new rather than terribly torn up and remorseful - or so I'm told

.
In your section on FN's career in "Finding Amelia", you seem to be hinting that FN may have changed careers because he was never made captain although he held a sea captain's patent; in your answer you seem to be implying that he'd have minded losing his star status after leaving Pan Am. Is there any indication that he did, or was, or are you just assuming he would have been? Navigator seems a bad career choice for someone who left his prior job because he resented always playing second fiddle. It seems to have been very much a background job, ideal for someone with specialised skills who was comfortable having lots of resposibilty while NOT being a team leader and NOT getting lots of attention. FN rather strikes me as someone like that. In photographs or on film he often has his back to the camera, or he's busy with something, or looks away - he doesn't appear to be eager to have his picture taken with AE in the way photographers complained George Putnam was. FN seems to accept being in the pictures as part of the background scene. He spent more than a month with one of the most famous, charismatic women of her time yet in his letters to Helen Day he doesn't gossip about her, or brag. His attention is on his job, and on what he notices travelling. The only time he mentions something she says, he's relaying a compliment of Amelia's about his young correspondent.
I find it surprising that FN found the time and space to write to Helen Day at all. (If he was pursuing some kind of career-advancing scheme with Eugene Pallette -whatever could that have been? A navigator's experiences aren't exactly an Oscar-winning subject- it'd have been much more sensible to get off more reports to him.) In the letters he comes across as a considerate person. He obviously knew Helen as a child and is eager to assure her that she's become a nice young woman, and always asks to be remembered to her family. He seems to be enjoying the trip and the new experiences immensely - for a man of his time, FN seems remarkably open about foreign customs and cultures; his letters to Helen almost entirely lack the assertions of cultural superiority that make similar contemporary accounts such disturbing reading nowadays. (That's really quite special considering that he seems to have had little formal education.) It's the kind of letters I'd find hard to write if I were preoccupied with unpleasant thoughts about my reputation and my career (but I'm not FN of course).
There's the matter of his drinking, and his leaving Pan Am under a bit of a cloud -maybe, he may just as well have been fed up with working conditions ther and looking for a more settled job that would leave him more time to be with his new wife, and working conditions and treatment of employees don't seem to have been ideal. Being asked to go on the world flight (there's no indication he applied for the job, or is there?) may have given him a gratifying feeling of "So there!", and he must certainly have been bitter about how his work relationship ended, but would he have felt a need to
redeem himself?
There's a telegram FN sent to his fiancée before the Luke Field crash ("Leaving 1.30 AM your time. Amelia has asked me to continue with her at least as far as Darwin, Australia and possibly around the world. Will keep you advised. Trip around world will be completed before I can return from Australia. I love you, Fred", qulted on p. 39 in "Finding Amelia").
Am I the only one who detects a tiny whiff of guilt in what he says? He seems rather eager to anticipate any objections his fiancée may have when he states that travelling home from Darwin by steamer would take quite as long as flying around the world. I sometimes get similar text messages from my husband's overseas business trips and they usually mean that I'll have to handle things we'd been planning on doing together on my own, which tends to lead to marital discord

. Perhaps I'm over-interpreting this but it seems to me that FN quite badly wanted to make the world flight. It must have been a once-in-a-lifetime chance for him - when does one ever get asked to make a trip around the world in a state-of-the-art new plane, expenses paid? To navigate the routes his colleagues wrote about surely must surely have seemed an exciting challenge to him - to the extent that he was willing to leave his wife of six weeks and take part in the second attempt (Of course we can't know how his wife felt about his job, but it seems likely that this by itself would have provoked a bit of discussion even if his wife didn't grasp quite how dangerous his profession was?)
As for feeling constrained to stay on the flight because leaving would create a stir in the press, FN seems to have written professional articles mostly, and there's also a newspaper article with a very matter-of-fact description of his work at Pan Am that I can actually understand

, but nothing for mass consumption really; this indicates that he cared about his reputation among his peers rather than about being famous (how independently famous was he, or could he have become, anyway?), and since AE's reputation was rather shaky among the professional flying crowd, wouldn't they'd probably have been quite understanding about his reasons for leaving if he'd chosen to?
There's a lot that makes me think he must have been in better shape than AE when they started on the last flight - physically certainly (there is no mention of him being ill or very tired, and from what he tells Helen about the rijsttafel, he seems to have enjoyed the meal as well as the experience), but possibly emotionally as well. He doesn't come across as desperate at all, he seems to be enjoying the trip and the new experiences as well as looking foward to getting home. According to what your witness says he seems to have been preoccupied about the flight to Howland Island, but that seems appropriate (I must have overlooked that account before, there's such a wealth of information on the Ameliapedia). The flight must certainly have been stressful, which perhaps led to even heavier drinking than usual with him. Still, he was doing something he'd successfully done before, and knew inside out. AE certainly wasn't.
I still find it hard to understand why someone who comes across as responsible and thoughtful to me should have decided to disregard the considerable risk of attempting to find Howland without decent wireless communication.There isn't really anything to go on except his personality (as interpreted by me) and what we know about people with similar experiences. People who work together closely on something unique and dangerous often seem to develop a special bond, even if they don't know each other well and don't talk much (which would have been difficult on the plane with the terrible noise the engines must have given off). Perhaps FN, who seems to have been a considerate, perceptive man and may have sensed how desperate and rushed AE was, and how very badly SHE needed the flight to succeed, came to feel she was a comrade or chum of sorts and that it simply wouldn't do to leave her to go on to Howland on her own (telling her she couldn't or shouldn't do something seems to have been a sure way of getting AE to attempt it - as when Wiley Post advised her not to fly over the Gulf of Mexico). Never to leave a comrade in the lurch seems to have been an important theme with men of his generation. This is, of course, wild speculation (and I'm not really happy speculating).
I very much want to believe that AE and FN wound up on Nikumaroro (I can just hear my former philosophy teacher saying "If you want to believe, go to church - scientific reasoning is about discerning reality not inventing it"!) because the evidence you've come up with at the Seven Site and your work on the artefacts are great and so is your work on the post-loss radio signals (not that I understand the technical aspect thoroughly). I just can't see so many coincidences piling up. But that still leaves the question of how the plane and the crew got there. It's always bothered me that AE/FN should just have headed off towards the southeast IF (but only if) FN (who was the crew member with most of the know how and experience) knew, or was convinced he knew, that they must be close to Howland Island. Gary LaPook's box search hypothesis seems to make much better sense because there were people waiting on Howland Island while the Phoenix Islands were very much unknown territory. It has the additional merit of being referenced in the literature of the time - that's important to someone like me who's been taught, and is convinced, that if one can't established a historical personality had a thought, the next-best thing is to establish that the thought was current at the time. But if FN couldn't have been convinced he was close to Howland Island at all,(which is what you seem to be saying) that changes things considerably. If FN was aware that his chances of reaching Howland Island without decent wireless communication were actually rather slim, this leaves him with a motive to look for alternatives early on, even if he didn't do it on a conscious level. It doesn't make sense to search for a tiny island in the middle of the Pacific then.