Your vibe detector is correct. Oliver is no longer doing custom builds. Though if you're nice to the guys in service, you might be able to talk them into doing some things afterwards. But there's a lot that Oliver did for me that I probably should have done myself. Not that they did a bad job, but fitting out the trailer slowly would have saved some money and headaches in the long run. It was just too big of a project for me to wrap my brain around at the time, so Oliver getting me 80% there helped tremendously.
The furnace is definitely the propane hog of the trailer. We've been out in December and gone through a tank and a half of propane in a week. (Also using the propane campfire, which is a hog as well). In contrast, last month in Utah with only a few mornings' use of the furnace, we only used half a tank in three weeks.
We gave a good bit of thought to induction, but I'm glad that we didn't do it. It would have been a big power drain and we've seen no drawbacks from the propane stove. If you're set on it though, take a look at a Volrath portable induction burner. It's expensive, but it's a restaurant quality unit that has a true simmer setting rather than phasing power like the cheap ones do. But if you're just going to boil water, get a cheapy one or use a Jet Boil - those things work better than any cooktop.
One note about the electrical, since you've mentioned two phase a few times on the 50a service: RV 50a isn't like your home 240v dryer outlet - it's 3-prong 120v, just with bigger wiring and a different plug. Also, most of the trailer stuff, including those marine heaters, will run off the 12v system; i.e., they won't impact your 120v.
I doubt if those hull heaters will give you much in the way of floor heat. Potentially in a small spot by the beds, but most of the floor sits directly on top of the fresh and grey water tanks, so there's no room for air circulation.
The shades do a good job of blocking light, but you'll want to address both the MaxxFan and front door window if you really want it dark in there. There are solutions for both, so not a big deal.
For the bed area, Southern Mattress will make pretty much anything you want, custom fit to the curves in the Oliver (what they make is better than anything Oliver offers anyway). If I were you, I'd do the king bed model, but get whatever size mattress you need from Southern, and fill in the remaining seat area with custom cushions. Then live in it a while and to what your storage situation is like. You can always swap out cushions for storage once you've settled on what exactly you need.
Another thought would be to do a 4" deep mattress and build a platform storage area underneath. You might also consider fold down counters on both sides, which would give you some much needed counter space.