Offer to add an addendum

and the addendum can be revoked in 15 days after move-in if both sides agree (in writing), which will essentially stick to everything in lease, but give you extra days to validate.  If either of you disagree, the clauses in addendum with stay with the lease.

On the addendum, put in the clause/items you want to include.

This is a "both side step back" approach in negotiation.

Of course, if they don't even want to do that, you may not want to proceed.  After all, working with what kinds of people is often more imporatnt than living at a particular place.