Honestly, I'd like to become proficient with foundation issues / mold remediation since most people seem to be afraid of these problems, long term I think it'll give me a leg up - but probably not for my first deal, as you said!

Every time there is a new violation or situation not covered in the lease add it to your nonstandard lease provisions and have all your new tenants sign/initial next to every one of them.If you want the cleanup properly done, I would have gotten a quote from a third party and sent the quote along with the notice to remedy nuisance or threat of harm.  

Although the house is on a street with many well-kept homes, it is next door to a less than well-kept home and 2 doors down from a condemned semi (fire damage, the town hasn't made efforts to remediate).

Bid from mold remediation company was $4000-$5000 for cleanup.

you should certainly add the extermination fee to the bill along with your person charge in order to remedy the issue.

Any comments would be appreciated on how to remedy this situation.

Was wondering if anyone has a remedy or experienced any trial and error ways of getting rid of it?  

I like the idea of a simple written agreement saying you pay to remediate and they pay if its new or they desire a more extensive rebuild but I don't know how you would write that. 

Sample belowTO:and all other occupants in possession of the premises commonly known asaddress___________________________________city of ______________county of ________ state of ________________________You are here notified that your account is delinquenct in the amount as detailed below:Rent due: __________Late fee due:_____________You must hereby instructed to pay said rent in FULL within 3 days or to vacate the premises within three (3) days of service of this notice or your tenancy will be terminated and the landlord will be entitled to all remedies relief and damages allowed by law including but not limited to recover rents, court cost, and attorneys fees.

If the only default remedy for the seller is to keep the buyer's (wholesaler's) earnest money, I don't see a problem with this verbiage.What do you think?