Is 25 day grace period (10 day after Notice of Default + 15 more days) sufficient to cure default?

Usually they are under rented and you can cure the deferred maintenance, improve the condition of the property, and then rent it out for a higher price. 

I'm not saying LLCs are useless but they are also not a cure-all. 

Send a cure or quit notification.

However, there are 2 ways to cure this challenge.

Do I "dig in" and start with a cure or quit notice to begin process after she's late OR do I attempt to offer cash for keys?

I'm not sure what is in your "CYA" disclosure, if it is from guru I'd run that by your attorney as well, they are not all compliant nor sufficient.If the seller has addressed all disclosures required and you have basically told the seller of the pitfalls of a sub-2 transaction, you may be ready to clsoe.......ask the settlement agent.Evidence of good title in ten days is pretty standard stuff as after you contract for the purchase, you have time for a title search and if there are problems you have the opportunity to accept the defects (if they can not be cured) or to continue to purchase.

@Jeffrey AlbaumIf water is from seepage and there is no exterior grading, other exterior issues or foundation cracks then the waterproofing system is the cure.  

There are ways to cure the foreclosure (bringing mortgage current, buying from owner and giving them something for their equity, if any) but you have to work with the owners as their lenders can't talk to you without permission or assistance from the owners. 

If the tenants have people living there who aren't on the lease/ aren't approved co-tenants, then you should send them a Notice to Cure or Quit (or equivalent notice for your area).