Zone4 - you might not be happy but disclosure wouldn't apply here. There is no code requirement that applies (that I know of) and so there is no breach. It would seem that installing LP for that purpose alone seems a little extreme. One option - which has cheaper run cost and install than the LP tankless option - run a recirc loop from that bathroom to the tank. Have it on a switch. You turn on the switch and the line fills with hot water. The pump is $200, electric should be $200 and extra plumbing is $100. You won't waste water and as long as you shut it off, you won't waste electricity to heat the water. My $100 charges of course are guesses and the plumbing in the slab might be more. The $200 electric is what it would cost me in NC. You can of course put the kitchen sink on the same loop and install a switch at that location too. Recirc loops are done all the time but where you need to be careful is the energy loses because they are usually left on nearly all the time. By having them on-demand with switches, you counter most of those losses. To help with some of this, I would definitely insulate the slab plumbing lines. Another option would be to do solar for the hot water and use a 80 gallon tank. If the incentives are there, solar can be reasonable. I paid less than $2000 all in which is about the cost of your 2 50 gallon heat pumps.

I don't want to talk you out of a water heater if that's what you want to do. However, a 40 gal heater should last about 20 minutes before you run out of hot water - based on 120 deg water, 105 deg shower and 2.5 gpm shower head. If either of you like showers longer than 10 minutes, really hot showers, or more than 2 showers back to back, a 40 gal won't work in your situation. I suspect that you are moving more than 2.5 gpm through the shower head. The best answer is to measure it. Fill a 5 gal bucket for 1 minute from the shower. If you have more than 2.5 gal, you have found your problem.

Here's a dirty secret...In general, most of the heaters are using many of the same parts and controls. There's some difference, but not THAT much difference. Bradford White is a good heater. They are only sold to professional installers (plumbers, HVAC). They cost a bit more because they are slightly higher quality - and because they don't have the pricing pressure of a big box store. They should have a good reputation because they are going to be installed better as a general rule since they aren't sold to the DIYer. Installers like them because slightly higher quality AND because the end customer can't go to big box store and see the cost. Look into Rheem or Bradford White -- the Whirlpool problems with their pilot light were handled really poorly and left a really bad taste in everyone's mouth... as I say, I don't know if the problem has even been fixed yet.

I'd really like to have sympathy for your situation... I'm just having a really hard time mustering it. A 70 gal "Asian soaking tub", like multi-head showers, are so wasteful it falls into the gluttonous category. You do have to think it all the way through. Don't install something like that unless you can do all of it. If you can't afford what it takes to make it work, then you can't afford the tub (or multi-head shower) It's like buying a Hummer and complaining about the price of gasoline.