After speaking with maybe 12 or 13 different lenders in the area (and being rejected by all due to the property type) I approached the seller and informed them, politely, that the property would not sell unless someone was able to offer cash.

Guess what, your first 10 offers probably won't get accepted, so get good at making them and accepting the rejection.

My key areas of rejections are: any past eviction in last 7 years, past due car payments, judgements/bankruptcies , utilities in collections, address/social security mismatches(I've caught a lot of scammers this way), fake landlord references(usually caught through simple county searches).

He had much higher cooling needs than us above his apartment  and rejected when we offered him a window ac and was literally driving our ac for his own needs.He even asked me during summer if there is any way to optimally adjust the pressure of air flow as per his needs in one of his bedrooms.Fast forward during winter he closed the vents in his bedroom and another one in the living room that comes before his bedroom.Now the same bedroom which once had a high pressure air flow has virtually no airflow makes me wonder if there is a duct burst or duct got pinched somehow?

In CA the buyer has 3 days to reject the property for any reason and get their deposit back.

I can guess why your offer was rejected.

I did that the bank closed the account an rejected the payments 

If a cash buyer wanted to put his property under with me (property newly renovated), could I flip it to a buyer with a traditional bank loan, or will the bank reject a whole sale contract?

I would ask the sellers for financial assistance for this repair, however, it appears that the sellers rejected the buyers that came before me because the previous buyers requested $6,000 for repairs.

If they are honest and it shows in the numbers I may work with them if everything else works out and if they lie this is an immediate reject.