Seahawks have a tendency to keep around marginal talent if they to be lights out on Special Teams. Which Marsh was last season.
With that said I think Marsh will be tried out at LBer to see if he sticks, hes a guy who I think plays better when he can maximize his athleticism. He touched around 270 his first year, was about 255 last year, and is reportedly 245 this year and his instagram pics probably agree. The guy looks to be in phenomenal shape, not much bulk on him where he can just max out his athleticism. His upper body and core strength looks better.
Kam, Marsh, and Webb have impressed me the most with thier pics. Thats not saying much but still they look like beasts and they seem dedicated to thier overall fitness in thier spare time.
I think with Marsh's DE/DT experience he could be adept at holding the edge, he hasnt been productive in the pass-rush role but I think he could excel at least as a 2-down assigment correct LB who would be stout in his run fits with his ample experience taking on Offensive Linemen.
Unless French can be tougher and hard-nosed, I think Marsh has the best chance to replace Irvin. With MiMo as the hedge if both of the others dont work out.