+Spd -Res Seth?

A 5 star one? I wouldn't fodder him for Fortress Defense. I'd fodder Ruby Sword+ to a highly merged 4 star Palla or Olivia who both make great use of that weapon.

If it's a 4 star one, yes, fodder him for Fortress Def.

Depends on your barracks, and your intentions. I have the same Seth, and planning on building him whenever I have the time. If you lack Eldigan, Seth is a pretty nice bootleg version of him. With +spd and a spd+ seal, he can also be somewhat offensive. I want a third or perhaps even 4th cavalry team, so if you're like me he's a pretty worthwhile red cav unless you have several of: Leo, Sigurd, Xander, Eldigan, Brave Roy, etc.