A late charge applies after 3rd day.If on time, the only further action is for me to move money to master trust accountIf not in account by 8th, I refer to U/D attorney to serve 3 Day Notice to Quit.

I would present all of the facts to an attorney then wait for their advice.I hear Massachusetts is pretty tenant friendly so I do not know if the following will apply but in most areas the tenant cannot simply withhold the rent due to repairs.

It would be totally appropriate to ask about termination terms in lease and request landlord to apply as part of your offer, if it's available.  

I'm asking this question because I want to start vetting investor-friendly brokerages now so I can be ready to go when I get my license.So, that said, I'm curious what the process is like when candidates apply to a brokerage.

They are selling you generic information that may or may not apply to were you conduct your business. 99.9% of time it is rehashed, recycled, misinformation.

If neither of those apply, did the tenant move someone in illegally (i.e., who was the 4th person)?

Same answer still applies: pull each file manually.

I know most banks want you to have at least 20% down, but does that apply with private lenders?

@William Rankins you need to go in and figure all of the cost of the rehab if it needs any, take that off of the ARV make any adjustments as you think you may need then take off the amount you want to make off of the deal and subtract that from the ARV and apply.

I do not think that landlord-tenant laws apply as OP stated that this is a sale with owner financing.