It looks like the best way to migrate my data back to the old Air is gonna be to delete my old user account on it, then use the Migration Assistant to pull the latest Pro backup off of Time Machine.
I just created a new admin account and hit the delete button on my old account in the Users and Accounts prefs. It took like two minutes for it to decide to react and bring up the “are you sure you wanna delete this account” dialogue. It probably would have taken a lot longer if a Dropbox accident had not deleted all my working files off this old machine at some point after I switched to the Pro, as it’s telling me how much space I’d need to save a copy of the account. I wonder how long it will take to delete it...
This is definitely gonna take a while. Guess I’m not getting any art done today.
Oh dear. My user folder is about 700 gigs and there is only about 500 gigs on the old Air’s entire drive.
*idly wonders if she can persuade the Apple Store to give her more than $1001 on trading in this Macbook Lemon Pro and get a new Air instead with a 1.5tb SSD for... less than what she paid for this damn thing.* Probably not. Worth asking. Not worth planning on.