missing ground?
 

anyone know where the ground wire that bolts near the dipstick originates? post engine-swap and a year of the car sitting untouched I cannot find it at all.

Hint: You already mentioned it in your post, it in fact bolts to the dip stick stud.

on 1.6s there’s a m8 boss on the head back there, but even they can just go to the dipstick.

Its on the plate between the trans and block I am 99% sure... I think that spot also shares a ground point over to the chassis, but I might be making that up.

This might be a dumb question, but have you tried tracing the wire from where it's supposed to connect near the dipstick? Sometimes following the path backwards can help locate the origin.