Out of curiosity - why the nut should be stainless steel? There is no contact between the nut and the aluminum terminal. So there should be no impact on the cell terminal related to the nut.
Regarding the seizing - do oxidized stainless steel nuts seize too?
Regarding the retapped cell terminal - what is the insertion depth of the stud? From the pictures, I can see 2 and a half turns and this would be 2.5mm if I'm correct. This won't hold sufficient torque. Especially with the quality of the retapped thread.
If it was me I would redrill for M8 stud and use fine pitch M8 stud. Proper thread locking glue (permanent) would also add some strength there. Or check what suitable welding options (spot, laser) you have around you.
And why not contact Eve directly for advice regarding the max drill depth? I know some folks here got information regarding the cell compression they recommended directly from their engineers. So it should be possible.