Since no one else has yet to respond, I'll be the first a take a crack at the long list of questions. Bear in mind, I own a 38, and the home waters are the Chesapeake so there are obvious limits to what I can contribute.
In a lot of ways the Nordic and American are two variations of the same choice. The two are quite similar in many ways. Both have hulls that lean more toward a planning hull. The Helmsman on the other hand leans more toward displacement while still being semi-displacement. I have not needed to do the math but I think you will find the Helmsman is a heavier boat per foot of length than the NT or AT. That weight will impede speed but assist stability, so you pick your preference in a world where you can't have it all. Leaning toward speed, there is more deadrise (V angle in the bottom at the stern) whereas the Helmsman is virtually flat there. The flatter section will make things more stable at anchor or in the slip, but the V helps the ride at higher speed. Again, you pick.
I can't comment on the Selene.
Beam seas are obviously the least comfortable point of sail on a trawler. In smaller chop the Helmsman will "squat" and plant itself harder into the water when above displacement speed since its hull leans toward displacement. That behavior reduces roll in chop. There comes a point when the seas are larger, or you are dealing with rollers, when stabilizers might be desireable. Others can better comment on where that line is.
Seaworthy? Well, it is a coastal cruiser, not a passage-making, cross the ocean sort of boat, and none of those you are considering are either. To me, its at least as seaworthy as the AT and NT, but again I can't comment on Selene. It is built heavily. Nothing flimsy about it. Helmsman once compared it to a tank, and I'd say that's apt.
There is at least one 43 with a large engine that likes to cruise it in the mid teens. Perhaps he might chime in. Personally I prefer 7-8 knots, for the peace and pleasure, and the economical fuel burn at that speed is a nice kicker.
Do the research but I think you will conclude you do not need fuel polishing with a Cummins QSB. The fuel pump delivers a lot more fuel to the engine than is burned, returning the excess to the tank. But in that process the fuel is filtered, so in essence you have a continuous polishing process happening in simple use.
However, quite a number get the Reverso system to make maintenance easier.
As a 38 owner I'd say my experience with build quality, order and delivery process, and after sale support is as relevant as anyone's. Arguably more relevant since I am on the east coast whereas support in the PNW is easier for the company to provide. Here is the honest answer. On a scale of 1-10, its a 10. Yeah, I know that sounds like a fan-boy answer, but its honest. I had Helmsman on the phone instantly within the last week when I needed to be walked through something. I think I'll stop short of giving examples that prove the point but if you get serious about a Helmsman and wish to pick at this a bit, feel free to reach out and we can do that off line.
Build quality? Better than I hoped it would be. My aging body is in revolt at the moment since I opted to polish and wax the hull and replace the zincs myself this past week before launch for the season. But that was a week spent with my face staring at every square inch of the hull, a foot away, inch by inch. After one season's use, mine is perfect. There was a time when Selene's had poor gelcoat. I heard that was addressed, but then heard of another such issue on a recent order. Do your own due diligence on what is true and what is internet rumor. However, what I know is true is that mine is perfect. Interior? Just go aboard one and judge the fit and finish yourself. Mine is flawless inside.
One thing about build quality is that its easy to roll into that the quality of the installed gear, and maybe one should. After all, choice of what gear to spec into the boat is something controllable by the builder. I think you are going to find, piece by piece, its all name brand gear that runs at the top of gear choices people gravitate to for quality. Helmsman leans towards quality over cheap as a rule. But should a top quality piece fail, can I call that a Helmsman fault? I don't think so when its generally considered quality gear. What you will have is their support in getting it addressed, which is all you can ask for. The best illustration of that is gear I personally selected, which was Simrad electronics. The Simrad autopilot hydraulic pump is generally considered to be the best there is, and many try to incorporate that pick into non-Simrad packages. Well, against all odds my pump delivered and installed was defective. Helmsman helped immensely in getting that sorted out, replaced, reinstalled, and tested. That is but one story that leads to the 10 out of 10 grade. There are more, but I'll stop there.
So would I order from them again? In a heartbeat. No hesitation.
The harder part is what do YOU want. If what you want is what Helmsman builds, you are in luck.