I didn't know what they were, but looked them up and found out that they come filled with nitrous oxide and are called "whippits" which you can inhale to get high (and just a bit of brain damage, which now makes total sense given his behavior). 

Yes, the involvement of your daughter is disturbing and totally not OK, I would tell the Mother (owner) that this behavior is unacceptable and if she is not sympathetic then don't worry about burning that bridge, it wasn't worth crossing.

Include a copy of the termination letter with your response forms to indicate your “no tolerance “policy for this type of behavior. 

Something to reward the behavior when your friends do send you leads

Now he's claiming I should only make them replace the glass and the latticework (basically repair the door) and I am not opposed to this, IF it can be done and I told them that.They're just making me feel unreasonable and I know that's what people do, they try to justify their horrible behavior, but it's really starting to make me sick (literally) with anxiety.

Furthermore, going off your display picture I am also assuming you are young (or look young).

The hero section is whats going to be displayed above the fold, which is everything in plain view without scrolling.

Some of the business dealings described go as far as criminal behavior and fraud.

Is this normal market behavior?

If your original plan was to reduce rent by $25/month, then you're basically willing to give back $300 in annual value as a reward for this behavior.