2) Does Fidelity open up a second FBO account in the name of John Doe, so that he can regularly contribute to 401(k) plan?

If you can train your tenants to change the filter every month, then more power to you........But in the vast majority of situations, that menial task is rarely done by most tenants.....kind of like having your dentist tell you to floss every day :) Some do it, but most wont...at least not on any regular basis for a sustained time.Must be different in CA....or my house...I change the filter every 6-12 months and its is barely dirty.

Best you could do is regular inspections.

Check out some of the great regulars here on BP, most of them work remotely, like @Daniel Hyman 

From rough calculations, it seems that it'd be a smaller capital expenditure than a regular property while still demanding high nightly rates for the novelty of it.Does anyone have experience as a tiny house host?

Right now, I can handle my three (soon to be four) properties on the side while working 60-70 hours/week for my regular job.  

Do you do regular upkeep?  

I understand if your are going to go into the mortgage business and begin financing people regularly, then you need to become a broker and get a license.

Have you verified that the filter was being changed regularly before the test was ordered? 

Since the earnings from these pass-throughs is classified as passive, it is taxed separately from regular investment income (portfolio income).