This time I took photos of them breaking all the house rules-leaving 1.5 hrs late, extra guests, spilled liquids on the floor and some unknown substance staining the sheets, photos of a cigarette box, cigarette butts, cigarette ashes.

In my experience, the only thing that truly works over nicotine staining and smoke smell is a true Shellac primer.   

After a tenant moved out and stains couldn't be removed just throw out and put a new one..

Eventually you will come up against diminishing returns (replacing two year old non-stained carpets, for example, probably isn't worth it), but these little >20% CoC ROI things add up.

But now if there is a new, big stain in the carpet, I would charge them to have it cleaned. 

I even had yellow stains in my bathroom, showed management and they denied it was from neighbors smoke of. 

I’d tear up the carpet and then have your contractor sand it down and then finish with a nice stain.

Just a thought...you could buy that 1/4" bead board and go over top of it and not mess with the trim if you plan to paint and not stain.

Ordered similar coloured siding and stained wood shakes to install, instead of extending the roof and adding timbers on the front deck.  

A little off topic but I've always wondered, is one paint brand more durable or stain resistant then others?