I'm considering upping my aggressiveness on remediating this issue with something that will cover up the color of the mold so it will indicate better remediation efforts to future potential buyers. 

Kicking out a smoker will leave you, likely, with extensive remediation costs and vacancy during that period.  

There are a handful of options that could remedy some of your pain.1.

This is a fairly common issue with older properties  where root structures create a dislocation in the line causing constant backups and the only remedy is replacement.

He has 40 in another.Greed.... the only remedy without paying is for the senior lender to foreclose all those liens out . 

Educate them, give them time to remedy their mistakes.

His complaint was addressed in a quick and responsible manner and the issue has been remedied. 

Some remediation (or mitigation) can be expensive; however, nothing must be a deal breaker… just leverage negotiation when you are aware.Roy

Moreover, contamination from gas stations can migrate considerable distances and an owner could be responsible for remediating the plume even if was not the party responsible for the original spill.

If there is a large amount of mold or potentially mold in spots you can't remediate, hire a professional.4.