If I start making modification, how would I ever remember what was modified for each tenant?

The paperwork is a hand written sheet of paper unsigned by both parties. that says "upon payoff/sale, subject to good payment bonus Gary (previous owner) will be entitled to a $35,000 return of principal/capital to reimburse me for prior losses" bringing the total balance to $198,000 ( which is much more than anyone will pay, so that he can foreclose and sell the property for profit) I attached the handwritten "loan modification".ARV: $229,000Repairs: $10,000Closing Costs: $10,000This seems like predatory/illegal lending.

If it is not then a general inspection of all units for accessory apartments (which used to be called an 801 general inspection for apartments) will be required and this could be costly and require major modifications.

That depends on your deal structure / offer, what type of loan it is, whether there's down payment assistance on it, how old it is, if there are arrears to pay back, if it's having/had a loan modification done, whether the borrower on the mortgage is deceased or divorced or soon-to-be deceased, whether you want to buy in your own name or a trust or an LLC, whether you want to actually have title transfer on public record or not, whether you're ready to decide to pay 2 insurance policies or one, .... and so on.

Now that being said, large development projects which have a long runway, and typically have leveraged capital from institutional markets, may have draw-down clauses, or even mortgage modifications throughout the draw process, simply because the total loan is so large and the draws are significant, but that's not we are discussing here.Second Q = YO. 

Installation of a DIRECTV System may involve modifications to the residence.

I disagree with some on the side of "wholesalers are unlicensed realtors", not because of legalities, but because a realtor's motivation is not typically, especially in a bull RE market, to price with the investor in mind, but to price as high as possible to necessitate a sale, thereby leaving many possible buyers priced out. 

Remind them that you could take actions against the dog and all the modifications, but you would like to keep relations amicable:(1) As the owner, you need access to the unit, so provide me with a key or remove the device.(2) you (the tenant) need to sign the Pet Agreement or remove the pet(3) I did not approve the security system but understand your concerns. 

There are at least a dozen ways to get forbearance, deferrals, and modifications.

A loan modification after 4 years of ownership and the amount is higher than original loan amount.