Lastly, is it possible to put a “seal” on a current shingled roof, should there be any leaks?

This leaves the tenants extremely frustrated, especially if it's a refrigerator issue causing their food to spoil, or possibly an issue with an appliance leaking water onto the floor, etc. 

Besides the doors being unlocked, he has left the gas on his stove on and has flooded the bathroom on numerous occasions damaging the ceiling below.

What about leaking roofs or accidents you can't predict?

Plumbing, Electrical, Gas,.

Trying to avoid spending 2K on a brand new one as we may rehab the kitchen and replace all the appliances in a few years.This is the exact model looking to replace. https://www.geappliances.com/appliance/GE-24-Built-In-Gas-Ov...It was on sale but have not been able to find anywhere that has in stock. 

Remote control and locable thermostates,  water leak detetors and main cut offs.   

This year I've had two water leaks, one of them is ongoing.

Do that an at some point you will end up buying a vacant lot that used to have a gas station on it, that still has the old buried leaking tanks and the oil change guys just dumped all their oil changes behind the shop, and you will now be responsible for EPA cleanup to the tune of 100Ks. 

He also suggested I bring out someone else to camera the system again to get a second neutral opinion.I hired Cable, Pipe, & Leak Detection and the technician that came out today was very impressed with the on-site system.