The x-tenant managed to move again and get a new car just this spring, so no motivation to pay us since it's apparently not crimping their style.

Also NEVER pay a GC upfront, ever. 

If you’re talking about paying over asking or having a larger downpayment: Those aren’t  expenses.

Does this mean I shouldn’t pay down mortgage with such high interest?

They have 529s to use to pay their living expenses and school tuition. 

Devin,Just a warning about checkbook IRAs: Self Dealing Activities will get your entire IRA disallowed and you will have to pay the tax on all your money.Self Dealing Activities are frighteningly easy to do.Do a Copilot search for "Problems with checkbook IRA plans."

So I wouldn't trust a bid from a Contractor unless you pay them, the other guys will just mail it in.

I'm guessing the management company would collect all rents, pay expenses etc and the individual LLC would just own the real estate.

What is the best legal structure to assume his existing land contract while either concurrently paying him or writing up a 2nd land contract to the seller past the 4 years on the current land contract?

You definitely do not need coaching on anything right now because you don't want to pay someone to make your decision for you.