Standardairconditioner
HVAC'ker
When I was agreeing earlier with Smooky about replacing the drain, I assumed that this shower may have been on a 2nd floor, and you could access it from underneath with a hole in the drywall ceiling.there's no way to get your arm down the channel beside the drain to tighten the nut by hand.
Now I am wondering how you are going to fasten the new nut that is supposed to be where "there's no way to get your arm down the channel beside the drain".
This showerpan is short of being ripped out the next time it gives you a problem, so this is what I would do:
Without the nut, use loads of Schluter Kerdi-Fix and drop that drain onto the pan. Kerdi-Fix takes a while to dry, so continue working by stuffing the calk gasket thingie in-between the drain and neck, exactly like how you pulled the old one out. Instead of lubing the calk, smear it with Kerdi-Fix instead, that will slip it up.
Kerdi-Fix is pretty cool and robust stuff, if you can avoid stepping on the drain, I'm 75% sure this won't give you any problems for another 5 years. You have your iphone on a selfie stick to inspect annually to forsee any future problems.