It's also important to me this is mutually beneficial to both parties.

Closing in 2 weeks, got my third-party inspection done before initial walk.

It's not that you cannot buy and sell properties between related parties. 

Typically sellers are not motivated to carry back financing for a property they’re selling - the reason seller’s DO it is because either they are obtaining a higher price then possible with a cash sale or sale to a buyer obtaining third party financing or can sell a property that’s does’nt qualifying for third party financing because of area, type or condition.Bottom line is that in my experience 98% of the time seller financing is utilized the buyer is paying over market price.

How can I have the lease nullified by both parties?

In Virginia, a Memorandum of Contract is a commonly used and legally recognized instrument to cloud the title when a party has an equitable interest in a property due to a signed purchase agreement.

To have peace of mind before renting it out, is there a third-party inspection service that can check the unit's structural safety and other aspects to protect myself and future tenants?" 

That said, without some written and signed agreement from all parties, your best legal protection is to make the check out to all original tenants.5.

Not a lawyer, but this part is confusing as they ask for 60 days, but in the next sentence it says "If the number of days isn’t filled in, at least 30 days is required." 

same goes with any investor buying notes  Making new loans  etc etc.. you do have license rules.. owner occ loans you basically cant make new loans without proper license .. you can buy owner occ loans but you have to have a 3rd party service them i believe.