+spd -hp female grima or +spd -res female grima

I would say that the +SPD/-HP Grima is bettter and for ephraim the +DEF/-HP

+spd -res will be better in most cases. The general rule is that fast units have hp as an extension of their def/res stats because they rarely get follow-up attacked.

Her hero page suggests an hp bane, but I think that a res bane is a better pick when combined with the speed boon (and only with the speed boon).

This is a question of play style. Objectively, I think -res is probably slightly better, but if you're like me and you try to squeeze out extra potential and tank more than one hit, -HP is better.

I don't think what I'm doing is objectively good. Experience tells me that she doesn't normally survive more than one hit targeting her res. Things might change if I can buff her with fortify fliers. Maybe.

On Ephraim, hard to say. +spd can be good on a non-DC tank build because with some buffing he might actually block a handful of doubles; more commonly you'll be using it to crack wary fighter because his weapon effect will cancel with wary fighter but he might not win a spd contest with 27 spd if they're not dumping spd (which might well be the case on a low-merge Effie, for instance). However, if you're using hone cavalry, 33 spd is usually enough anyways, so I don't see why not take the def boon. If you're on the galeforce build and you're not using desperation, it gives him better survivability for the second opponent, especially if you use it with sturdy blow (which is my recommendation over death blow if you're using hone cavalry. 2 atk probably doesn't secure that many more ORKOs, but it does increase the chance of OHKO which would cause issues for trying to activate galeforce. Better just grab some extra def).