+SPD -RES LA!Lyn or +HP -ATK B!Ephraim

Nope, perfect IV Lyn will be much more valuable to your team. -Atk is unusable in a vast majority of cases aside from when it's so low already that you might as well trash it even further. (ie. Sylvia, Azama)

Inherit ur special fighter to lyn and ur good to go

Ephraim will blow most things up with the ignis and he has a forced double on both phases. Lyn with the right build will spd double most things on either phase and the glacies will also blow most things up, especially when she targets res, which is the lower defensive stat of most units. The play style is different. Your choice.