Who to give sturdy and mirror stance?

For Sturdy Stance, if you're going for a QR set and the unit has good ATK and DEF, then you can fodder him off to that unit. Otherwise, keep him because Ephraim is still a great horse lance cavalier.

As for F!Morgan, if you think you'll be fighting more RES damaging units, slap that Mirror Stance on someone with high RES, although you'll be really sad if you do it on a range unit and a dragon unit attacks with a refined breath. Blarserpent would be perfect for LA!Lyn, though I'm sorry if you don't have her, since LA!Lyn is far by the best candidate for that weapon.

I find stance skills generally inferior to bond skills because they give 1 point less and can't work on both phases. The case is not the same with blow skills because it's harder to position for the bonds to be active when I need to advance into the enemy, but when I'm preparing for enemy phase, it isn't that hard to activate the bonds.

So L.Ephraim is a merge, and F.Morgan is either a merge or blarserpent fodder.

On another note, if you're using the enemy phase H.Jakob build, I prefer close counter. His defenses are quite thick to begin with, and guard bow with ward armour is generally enough for me. Add a close def seal, and he's sitting in the low to mid 40s in both defensive stats with over 40HP to boot.