The former use as a auto repair could present some concerns and you should find out how the former heating oil tank was abandoned (removed, closed in place, etc). since you are converting this building as opposed to demo so you wont be excavating impacted soil, the principal concern would be potential for vapor intrusion since you're planning on converting to residential. this can be addressed with a system similar to radon system.

Let her be responsible for getting his signature.

However, your tenant could successfully have the eviction tossed if she convinces a judge that the eviction will lead to her being homeless.

After several days of heavy rains, a tenant (who we have already had multiple problems with) sent a message stating that her roof leaked and ruined her bed and clothing.

The Issue At Hand: I got a call from the downstairs tenant stating water had been leaking from the ceiling and onto her bed. 

Batted insulation has a vapor barrier that keeps moisture away from the drywall.

Her being in quarantine does not stop an online transfer. 

I'm wondering if I need to act fast and when am I on the hook for her being a resident here.

I'm seeing this pattern of her being charming when she wants something, a (Beep) when she doesn't like something, and she's kept her head down for 5 years and has only had 2 small rent increases and has a deal. 

Being the only one on the mortgage doesn't invalidate her being on the deed, it just relieves her of the debt obligation.