(See Idaho Code 63-1005 – One-year possession under tax deed conclusive as to regularity of proceedings.)That makes it sound like you have to hold on to the QCD for a year to clear up any potential issues with it, but I haven't seen an indication of that anywhere else - including Idaho Code 63-1005 mentioned there.

In my opinion, she has not legally surrendered her possession of the rental unit if she has not returned the keys. 

While this particular property may be immediately rent ready, sometimes you’ll take possession of a unit and want to do something before you rent it - i.e., cleaning, carpet shampoo/steam, paint etc.

Auction requires you to take the property as is with no guarantees of possession and you cannot access the property.

We closed in September and I took full possession last week. 

Focus on areas and properties that allow you the opportunity to take advantage of information, manpower, or capital that competitors may not possess. 

He has not yet taken possession, and the lease has not started. 

I was so infuriated I started throwing their possessions in the front yard before they came home (illegal I know, but didn’t then) and ran them out of the house in 3 days.

Adverse Possession does NOT require trespassing.

I would prepare an agreement for mutual rescission of the lease and refund her $ once she signs it... and would do everything in my power to prevent her from taking possession.