Should I get a debit card for each bank account?  

The tenant paid the rent for the month of February to the property manager, then the previous owner instructed the property manager to give me the entire rent of February.Everything seemed fine, but a few days later, I get a call from the tenant asking why they had to pay February rent, when they already pre paid for February at the beginning of the lease.

Make the seller's monthly payment directly to the lender (by check or ACH debit)?

You might consider joining Pre-paid Legal.

Local lenders are usually full-recourse, but can be prepaid early.

I have seen $9,600 due bills in properties closing (due to my clause debited from the sellers closing statement).7) I buy houses on the market (MLS), but I most like the off market deals (once you start investing hard with cash those deals start appearing...those are the best).8) I NEVER EVER feel I am done, I can still fail and take all the precautions to avoid it.

They would have had to pay using a debit card with a absurd fee. 

The state Realtor association has a fine legal hotline, whose advice I bounce off of the advice from the excellent Prepaid Legal attorneys.Sharon gives outstanding advice on this topic.

One of the contingencies is that I agree to pay up to 3.5% in closing costs and prepaid items.

After all debits/credits I'm all in at $305.