I had to explain that while they were noisy and ugly as sin, they posed no threat to humans.

Financially speaking the threat is that capex will start to exceed cash flow a few years into the investment - aka money pit.

let them direct their threats to the lawyer he'll tell you the ones to worry about. 

No matter what, tears, threats whatever.But, I also would tell them I'd give them their full deposit back on move-out day, in cash, if they leave the place empty and clean.  

Another reason in case might get dismissed without prejudice is the landlord is a weak scumbag who realize he doesn't have a chance in court against an astute tenant and he doesn't want to have to pay all the penalties for turning off utilities and making death threats to get people to leave.

The other allegations are not factual.Then, Mr Reynal sends me emails insisting that I buy his duplex back for 20k more than I sold it to him for under the threat of impending legal action.

Threaten to sue the agent and the seller if they don't return your earnest and void the contract because they breached it.I think you could actually win on this basis, but usually just the threat of a plausible winning lawsuit is all it takes, in my experience.I have great luck getting folk to settle with a tough bluff backed by a potentially winning case.

@Roshan K. is exactly right - treat the daughter exactly like you would want to be treated - or to drill down even more, threat her exactly the way you would want YOUR daughter treated in the same situation.In addition to the PR hit you'll take if you enforce the guarantee, you'll have to figure out how to keep your own conscience from screaming bloody murder at you in the middle of the night.

The only way it would help your cause is if she acted on your threat and moved on.

Perhaps a visit to the water company together for a scheduled appointment with a supervisor is best, but don't know if this is feasible with the coronvirus threat these days.Whatever balance is outstanding after all these efforts I would probably propose to the tenant to split it after pointing out the clause in your lease.