なにか can mean something as well as somehow while なんだか is always the adverb somehow. なんか is a less formal, more colloquial version of なにか with its に changed to ん (cf. 撥音便)
Although だ should be etymologically the copula, but as you can see in the link, it would be simpler to consider them as independent words on their own.
なんだか悲しい気分だ = なんか悲しい気分だ ...
なんだか is three words なん + だ + か but it's become a set phrase and is used as a word itself pretty much to express an idea like somehow, somewhat, or rather.
It's rather cold today.
なんか is different with a couple of different uses. It's usually an informal version of なにか or など.
I want something cold to drink.
I think your understanding is (somewhat) wrong at the first place. I would say:
I've never seen a field/crime scene where another hero is as successful as I am.
"他のヒーローが俺くらい活躍をしてる" modifies "現場" to convey the meaning "A place where other heroes are doing as well as I am..."
I don't know the context so I don't know if there's a ...
As far as I'm concerned, relatives clauses are placed before the noun they modify unlike in English. 「他のヒーローが俺くらい活躍をしてる」 modifies 「現場」to convey the meaning A place where other heroes are doing as well as I am. As for your second sentence, I think it conveys another meaning because で marks the location of the action. Therefore, 「他のヒーローが現場で俺くらい活躍をしてる」means ...