In my humble opinion, It's completely unnecessary to recreate the wheel and repeat rookie mistakes.

Now that we have that out of the way let’s get into the details on how to do this.Seasoning has reared it’s ugly head more prominently than ever recently.

Or is it too grey and in the rear view mirror for an AirBnB? 

However, older homes can have a burden of under $100.00 a year versus $1,600 a year in a bad neighborhood.There is a great book called "Deals on Wheels" by Lonnie Scruggs for more information on flipping homes.

Every other tiny house I’ve seen looks like a narrow repurposed tough shed on wheels for oompah loompas.

The best view I have of Baltimore is in my rear view mirror 

The pump is usually located on the return side, while this can be located on the supply side the diaphragm and expansion tank is also on the supply side not the return.I also don't see the t&p valve but I suspect it is causing the water sitting on the floor, it should be on the water heater side towards the rear of the unit.

That is work many aren't willing to do, so you are well ahead of the game.If you want to perfect your sourcing game and generate the best leads for the smallest cost and smartest activities, I think you should find a wholesaler (maybe @Lydia T. who has given you some great advice) so you don't have to reinvent the wheel.

You should be looking at lots with either 3,200sq feet of land area or LR zoning, ideally with alley access (for the DADU, as this removes the rear setback requirement an just makes livability/accessibility for the cottage much better.I know you are an architect and may want to design your own, but there are big cost savings to using an established and repeated design like those utilized by MyKabin.com

It would probably use wheels every few feet for the floor (no carpet would work) though tracks on the floor would be more stable, with a track every few feet on the ceiling.