I am looking for book recommendations on how to determine my investment strategy/my Crystal Clear Criteria.

I am meeting with a realtor to determine the property value as is and with some rehab.

How much proof is required and who determines it? 

And yes, hire an inspector or even a contractor to determine the cost of the repair, if the number still makes sense, do it.

I’ve tried many of the free available calculators to determine cash flow and i come up with about $150-200/month after accounting for current mortgage, taxes, insurance (not landlord policy yet, is landlord typically more expensive?)

Cost will not vary much on paper but in real life how they  screen, reduce turnover, handle maintenance, and evictions and rent increases will truly determine their cost and value.

Any advice on changing markets as a realtor, as well as determining the need for agents in a particular area would be greatly appreciated.

Seems like the size of your portfolio will determine which option is best for you.

I'm not concerned about selling a home with a properly remediated sinkhole along with the appropriate engineering sign-off, my question is, does anyone know of an engineering firm or company that can further investigate the sinkhole to determine exactly where and how large?

It's a bit tough to say since I don't fully know the situation but I think you need to determine what your goals are as well.