Especially with older buildings, finding the cause of one problem often reveals other problems, and solving one problem often creates a new problem...it's sort of like a game of whack-a-mole. 

Explain to your tenant that while you're trying to help them because you're an investor in the neighborhood these sort of complaints are beyond your property line and beyond your control.

You should consult a tax professional to help get it sorted and set up a strategy.

There would have likely been a public notice of some sort as well if a major change was going to be made.

I’d love to hear who you’ve used in similar situations, or what sort of rates and terms could be expected.

I am undecided if I should repair it and rent it out or should I  repair and sell it ( fix and flip) sort of.

Find local RE groups and join them all till you sort down to one or 2 local RE groups.   

Often times this is a "ask for forgiveness than permission" sort of scenario.

If your goal is assume a loan, I would offer the lender some sort of compensation as they are generally only paid if they write your loan, which is not the case with an assumable loan, as the current servicer will be underwriting and providing the new loan.  

Might be cheaper to run a vent of some sort.