Vetting tenants is essentially the same as regular tenants, but expect lower credit scrores, bankruptcies, etc.  

The credit check is likely pointless if the corporation is paying the rent, but it is an apartment building so I feel like everyone in the building should have gone through the background check.4.

I have been working with a credit union to get a HELOC to allow me to tap my equity to buy additional real estate.

The other option I would consider is regional credit unions that hold onto mortgages/loans and don't sell them to the secondary market - they are "portfolio lenders" and have the ability to be more flexible with their terms.

Verifying last 2 years of rental history very important!

Any recommendations for a website that has worked well for running Credit and Background checks? 

It's a different kind of program where we don't even consider the DSCR on the property, we mainly just look at the borrower's credit, their investment experience, and the property itself (where it's located, value, use, etc).

what is the best way to fund renovations on a flip project (other than cash)hard money lender, line of credits, credit cards?

You've already gone above and beyond, but it seems you just dig up more problems.When you run a credit/criminal background, the credit report should list the applicant's current and previous addresses.

If you have access to a line of credit on your primary home, that could be a good option.