Sacrifing extra Lena for her weapon

Can you do it? Yes.

Is it worth it over his prf weapon? In my opinion, no. I usually like to keep a unit's prf weapon unless I'm trying some odd build.

on a dedicated defensive build, like the one you posted here, Safeguard would work well. You might miss out on some one round KO's from the reduced might, lack of threaten def 3.5, and aether's length CD, but he'll certainly appreciate the +2 Hp

eckesachs is a little too good to pass up for a kinda eh regular weapon. his refinement gives him threaten def 4 basically on top of distant defense 3. zephiel benefits a lot from those skills and any other weapon will dull in comparison tbh

Eckesachs is a great weapon and definitely better than Safeguard+ in my opinion.