Could I put an electric rinnai in the bathroom or a gas rinnai boxed in on the exterior? Do you think the plumbing would work if put the tub on a 2 - 5" platform?
Electric tankless systems need SUBSTANTIAL power going in to make a decent flow to fill a tub, especially where you are on that mountain, the wintertime incoming water is likely to be near freezing. You'd probably have to upgrade your electrical service and run a quite expensive cable to supply the thing. Since you're planning propane, you wouldn't need to address the gas pipe size like you might if it were NG. How is this building heated? If you have a boiler, had you considered an indirect WH?
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014