You under estimate tenants' rights too much
If I were the tenant, I would just request to break the lease, but I would RESERVE the right for any further legal actions. This is because --- I have evidence of mold. It doesn't matter if the landlord directly caused the problem or it was a natural disaster. It is landlord's duty to provide a safe and healthy environment for tenants. So if tenant found mold they have right to stop paying any rent until the problem has been fixed by professionals. Secondly if they can prove they got any health problem caused by mold, they can sue you and your insurance company needs to cover it at least partially. So, as tenant they are at advantegous position. How can they be afraid of you and sign such stupid "no suing" contract to break lease ? So if you tell tenants " You can only break the lease if you don't sue me.", they can just go ahead to sue you. The end result will be -- They will be allowed to break lease, and your insurance may have to cover their health cost. Don't threaten them like that. You can tell, they have already sent you letter and it shows they know the law and they are not stupid.