Got 2 copies of Myrrh, which IV is better?

Depends tbh. If you give her buffs, the speed +4 can help her dodge doubles, which is good for tankiness. But if you want to maintain her player phase capability, +def is the clear option.

EDIT: Assuming you mean Halloween version. Otherwise, +def is still better I'd say.

I suggest +def-atk

The first is a bad boon bad bane, the secons is a good boon bad bane.