The seller claims to have only collected rent each month and has no knowledge of the number of tenants occupying the units. 

I don't think I've heard of a GP charging a refinance fee, I would guess it comes out as an operating expense in the LLC so not only does everyone in a way collectively pay for it but you can write it off.

Speak in terms of “We” the collective group.

Is it acceptable to ask for a bank statement verifying rent collecting and or a tax return?

If rent collection is an issue, collect rent twice per month.

They ask you what you want to do and follow your order and collect $.

With that being said, would you try to collect the rent until the end of May, and issue 7 Day notice to cure if it is not paid?

The regular numbers you might collect for Cap ex and Maintenance might need more of a contribution than a home without settling issues. 

After refinancing, offer tenants owner finance and wrap my own mortgage, collecting a 10% down payment on the new ARV.

For 10% of the rent per month, they'll usually handle everything from marketing the home, showing it, drawing up lease agreements, extensions, rent collecting, maintenance coordinating, etc.