We use Myrental.com to do all our background checks and collect ID and pay stubs from there.

They're not going to close the deal without the money for current taxes due being collected, so you don't have to worry about it.

I visited Atlanta many years ago and I knew this area had a lot of potential for appreciation (before I knew anything about real estate) - one suburb that I was really impressed by was Alpharetta.

Worse, they could put the collection on both of your credit reports, which could show for 7 years even if paid.  

If you have positive cash flow with the HELOC, you can wait and collect cash flow until it sells or sell at a loss and move on.

They would not do anything and just collect the check for the sale.

If the state has held a Certificate of Sale less than three years from the date of sale by the county tax collection official, the Certificate will be assigned to the purchaser.

Unless she's breaking some rules, don't do anything but collect monthly rent. 

Any best practices on collecting past due amounts?

Right now, this figure is holding steady at an impressive 97.1%.