Maybe try visiting and showing your face so you're not just a random OOS investor if you feel they're taking advantage of that.

Any other ideas on must haves?

I have seen "noob's" have issues with, say, a 5br home and trying to get that payment standard from a 3br voucher holder.

Try crimemapping.com - it's not available for all cities but for the cities it reports for it's great. 

I'm trying to understand how the financing works; Let's say I get a mortgage on the property, but only own for 6 months, as I've purchased, renovated, and made the sale to the next owner.

I'm trying to figure out what renovation costs I should expect for this type of project.The real end goal for the duplex isn't necessarily profit.

I try to offer solutions to their house problems.

They tried to negotiate hard most of the time threatening they will take legal action by saying few items were not disclosed.

Have you tried talking to him and asking him to take better pictures? 

If you're trying to avoid a balloon payment - a long term DSCR would be best for you.