is -spd +res leif a lost cause?

I would bench him until I got another. Just not worth it too fodder him off. And his B skill is very useful for a lot of modes

He's not a lost cause, lol. If you want to use him then use him. There are plenty of ways of buffing a units stats.

I've been playing around with Darting Blow 3 seal and I think -Spd units are still salvageable if going full PP.

It somewhat sacrifices your EP and other potential seal options, and may feel a bit underwhelming but it's not as bad as it seems.

Going something budget like Fury 3/Deathblow 3/Steady Blow 2, S Drink, with Hone Spd/Spd Tactic support from an ally, and Darting Blow 3 make his Spd a lot more useable.

Y'now,Giving Him Barrier Blade And Deflect Magic,He Actually Survives Rein.

Yes and no.

Theres many reds so hes outclased with bad ivs (even with neutral ivs)

Hes still more than usable tho. He only needs reposition, bonfire and darting blow seal. He reaches 45 spd and 40 def on initiantion, with a bonfire proc thanks to his b skill. I wouldnt recoment inheriting more than that tho

Drive atk is old news. Only if ur really desperate.

If you're going to fodder him, consider using the steady blow for a watersweep Saizo. The idea of that build is that he has the def to take counterattacks from anything that hits his def.

The other thing is that he can be useful for some interesting infantry pulse stuff because of his B-skill. His HP can put him somewhere in the middle of the HP range of an infantry pulse team (guys like Bartre or merged Azama are in the upper end of the range; squishy mages are in the lower end) and he can pre-load a pretty nasty ignis if you can give him two pulses and QP. You've managed to avoid banes in the two stats that would affect damage for that strategy.