She sold it off market for 180 a few weeks later.

Then you can put the 100k in there, and its treated as putting money towards that business (working capital) and no one needs to pay tax on that movement of money alone (though there are tax implications later down the line).

A formal agreement would also be smart to avoid any confusion later on.Sounds like you’re in a good spot with the $80k to invest.

A few of them came back years later and paid everything they owed because my claim was preventing them from qualifying for a home loan.

Sure there are people who have done it but like Joe mentioned above is why your rate on HELOC is probably higher than your first mortgage so you'd only do it if your balance is really low and near the end of your term anyway so you're just getting it paid off quicker (like the last 1-3 years of payments left) becaues you want to get ride of the monthly payment and improve your cashflow now versus later.

Then instead of trying a little portion and letting me know so we could change it, he had the whole house painted and told me 4 days later, the paint is not good.

While you may be trying to do the right thing, if you buy the house from them below market and they find out later on chances of you getting sued by a pro bono attorney is very high.

LoL THEN they got another round 2 years later. 

I would never rent to any of them by choice and any I inherit in the future I would make every effort to get rid of sooner rather than later.

They "suddenly" look like geniuses 30 years later.1.