That's pretty much it.
Personally, I'd not deauthorise my Mac iTunes library (or decommission it in any way) until I had the PC account running. iTunes can be a PITA on windows and IIRC you can authorise multiple machines (I think it's 5) simultaneously, so no need to deauthorise first.
Also, before doing anything I'd make sure I went through the library on my Mac and made sure it was set to 'keep library organised' and 'copy files to iTunes media folder' so you know you've got the whole lot when you copy it over.
https://support.apple.com/en-gb/HT204668
For question 2, I'd keep the mac for when she decides to come back