Just curious, with Odin (or someone looking like Odin) having returned to life, wont that mean that Surtur can return as well now that he's not being occupied by the struggle?

Yes, it is Odin.  And yes with is return it means Surtur can return as well.

But how do we know? The Odin thats returned doesnt appear to have the mannerisms of the Odin that Thor encountered during the Thorsleep. That Odin actually appeared to be somewhat noble and likable (breathing life into humanity because he found Eath beutiful, but empty), but the one in Fear Itself acts like a deranged grandad that will burn Midgard before making a battleground of the remains.