Trash IVs, keep or toss?

I'd keep them both if I got 'em, always good to have spare units for merges. Do you have a need for their abilities? F!Morgan had a few good ones, last I checked. Myrrh comes with Hone Dragons, so that can be useful for Dragon Emblem.

That F Morgan is basically a neutral S Corrin +3 HP -1 SPD -6 DEF +7 RES so totally workable and better at mage tanking. I wouldn't invest in her over a decent IV S Corrin of course.

-DEF Myrrh is painful, but she is still at the top of the meta. She can still run DC/Iote's to counter Brave Lyn and Reinhardt perfectly. Alternatively you can use various skills like Fury and Close Defense to patch up her defense to make more use of Great Flame.

Myrrh does not care about IVs.