However, the City took a very strict interpretation of the code and ruled that the permits obtained by the prior owner (the seller) were expired.

So if your property is not placed in service until 2025, then your bonus will be under 2025 rules, i.e. 40%.

If this is a house listed through MLS, it used to be that the amount was paid for by the seller...but rules in the US have been changing lately. 

This is 100% spot on.Absolutely leave a comment.If it violates Vrbo or Airbnb rules then ask to have the review removed.

I'm concerned with how to enforce any rules now that he's trying to skirt the lease.

Most investors follow the 70% rule of thumb (ARV x 70% - Rehab cost = offer price). 

So you're asking a tenant to use a system you have put in place, then when it takes a couple extra days you want to charge the tenant - regardless of the tenant's intent and the fact that they are following the rules you put in place.

Analyze deals like a pro using the 1% rule and understand operating expenses before making an offer.

It is borderline breaking Airbnb rules. 

There might be some exceptions when it comes to oceanfront land, where you might only be able to lease but that's the exception rather than the rule.