I wouldn't nerf return; i would improve its counters. If current efforts were improved it ensured there are cards aviable that can't be returned, and a field in buildup wouldn't have to be crippled so much. Something as simple as a lvl2 (can also dodge Shade open) that is grim immune.
The +1Lvl increase units could use a second look aswell. Those Skills exist almost entirely to stop this return grimoire from returning them, yet it happens AFTER the grimoire openingphase.
I wonder about the possibilities to give Rankups to these cards instead. A Rankup "this unit gets lvl+1" would happen before the return grimoire, and thus actually ensure the card will not be returned.
It would be very nice if grimoires could get a rankup effect aswell, meaning that its openingskill goes before other grimoires if the rankup requirement is met. For grimoires i don't know if it's possible to script, but i can imagine it being very simulair to giving the grimoire an Agi value. (like how openskills on unitcards go in Agi order)