I can't figure out what the principle balance will be if I choose to just reinstate the loan.  

You can also invest into stocks, bonds, and mutual funds as well.

I am trying to work a lease option deal where the tenant buyer gets $200 a month out of their monthly rent to go toward the purchase price of the home once they exercise the option. I have heard that a rent credit can...

(if you have a specific resource you can point me too I'd appreciate it, info its never enough)Based on those principles I've found a few properties that, on numbers, seem to make good rentals(CapRate easily over 10% and CoC over 25%) but I know there are other pieces that make the puzzle.Once you find a good property in numbers, what else you normally look for?

Buy deals that the numbers work today and have money in reserves for tough times.I have never heard someone say they are not investing in their 401k because the stock market is overpriced.

I pulled some extra cash out of the stock market and want to diversify into real estate.

If the rent I offer is a better deal than others in the market, there is a better likelihood they will stay; be aware that some will move based on "principle" - mad because rent increased more than they would like, and they are willing to even pay more elsewhere just to show you you can't push them around  ...

If I were you, I'd pay rent to yourself every month ... either to pad your maintenance fund, or to pay down principle faster.

Is it sales principles or math or how to properly hang a sign?

Don't speculate - save the speculation for the stock market.