What is your relation to the tenant?

Under those conditions, and based on your relative level of experience, you probably need an attorney.

What happens if the lease you type up for your relatives property doesn’t hold up in court?

Just because no income in 2017 doesn't mean you just get to choose to not reported your related expenses. 

Depending on your relation with the bank, they may lend you 50-70% of the loan balance.

If you go that route you need several documents to protect you and effectuate taking over mortgage payments (POA, notice to release, etc. )If you only have a strict 90 days to do this then in your agreement that should be spelled out but several things depend on your relative and how secure he wants to be in the fact that you will basically hand the house back over to him at the end of 90 days if you haven't sold it.

Moreover, if you decide to rent the property to your relative, it must be at the market rent, you can't give him any concession because of the personal loan he provided to you to pay off your 401k loan balance.

Any profit split varies depending if your relative just gives you money or also helps you physically. 

just take a QC  and transfer the legal description from current deed.. and have your relative deed to you. and if there is a place for consideration just put 10.00... its on the county transfer form that you need to describe its between relatives not on the deed itself.. at least I have never seen it on a QC and I have done them in many many states.

This means YOU occupy it, not your relatives, co-workers, renters, or bums off the street.