He's been a huge thorn in our side, threatening to file for guardianship over her and/or try to have her declared incapacitated if we don't let him stay in the house. 

You could challenge that they broke their lease and they don't live there, but all they would need to do is show up in court and say.... no Judge I live there.I agree with @David G probably would be best to have an eviction attorney take a look at your lease and see what can be done. i use these guys landlordeviction.net ask for David

Do you declare that income? 

It's so much, that we see at least 1 per month out of 100 units, so I have instructed her she will take everyone or anyone to court for vacating early in an attempt to try and help her put her foot down, and control the issue.She followed the process and took a few people to court, and at least one of the cases that I am currently aware of was dismissed by the judge on the bases the tenant moved out.

Really, anyone can do it if you apply more than some adherent cash flow now calculations.I even think that there are some guru classes that seem to even ask their students to look for City Planning Developments, etc. so that they can buy ahead of it.When House #2 is only analyzed on a Cash Flow basis and you ignore all other aspects of what makes a property valuable in the future, it's more like judging a book by it's cover.

In a ballpark figure, how much income would I be declaring from the sale if I do that? 

We will let the judge decide as we already know he voiced his opinion regarding this. 

As part of your eviction, you will want to ask the judge to award a judgement for any rents due and eviction costs.

Can a urologist declare a 50 foot python to be a ESA for a patient?

Don't worry about clouding the title or filing a lien get a judge to force them to abide by their contract.