However you do it, the goal is to offset your mortgage as much as possible while you live there, you bank that savings for the next purchase and repeat every year or two.

I’m looking for a way to document all basic and financial details of each deal we purchase (downpayment, how we funded it, lenders used, mortgage, interest rate, rent, cash flow, when it was purchased, partners included, etc).

You can do that by just adding his signature and date at the end of the existing lease.

@Rick PozosNot everyone follows the rules when STR'S are banned and sometimes existing ones have a period of time to shutdown.If not enforced some people will run the risk till penalties or worse are enacted.

I spoke with my mortgage broker and she advised as long as the numbers make sense, I can still purchase an investment property. 

The market I’m in is great for renters (but the quality of renters can make you pay attention to their applications)  I’m rehabbing the 3rd house now, plan to remodel the 2nd house after I’m complete and then unsure if I should:-obtain traditional mortgage then should I cash out refi and find more property?

. $3500/month payments for mortgage and interest payment that i need to pay on sellers behalf. 4-5 month fix and sell time line.

Let them know they will not be able to buy a home, get a mortgage, rent in another community, and could jeopardize their current credit card limits.If you are at your wits end and you know they do not care about credit and are ducking you then report the lates. 

Would a piggy back mortgage be worth it?

If your not adding on to the home, or more specifically, adding a bedroom, you should be just fine with the existing system (they won’t MAKE you replace it)... that is to say it should be fine as long as it’s functioning correctly.