I heard that the tax law no longer allows us to write off the HELOC interest rate. https://www.irs.gov/newsroom/interest-on-home-equi...  

In terms of legality and contract law, doing anything other than stated terms would be breach.That said, you have to decide if you're going to "follow the law".

The seller wants to back out, solve his problem or walk away.....we don’t know what your contract says about default on the seller’s part, or what your state law says.

The closing company/law firm will handle all the paperwork and get your your payout.

I have support from my family(in law) that are interested in starting as well.

My understanding is primary residence are allowed to built ADU (Popularly known as In Laws unit).

His sister in law is my REI realtor, his dad was my dads partner in multiple REI deals and is my commercial broker.

You need a good source for local landlord tenant laws, a  reserve account for the unexpected, setup a payment tracking system, find a handyman, set expectations with your tenant about what constitutes a valid emergency, have the tenants sign a "walk through" list with photos of the condition of the house, make sure they have renter's insurance, 

Luckily we have been in the industry from it’s conception and have followed the constantly evolving law changes in state and aperitif counties.

I remember plenty of railroad track embankments that were up for sale (railroads were exempt from abandonment/sales by law, but the slopes of the land on either side weren't and they always popped up - of course completely useless as an example).