～にも

一橋が裏で糸引いてたのは明白だが
罪を問うにも やつらは表向き なんら手を汚してねえ｡

Which usage of にも is this? にもの意味 - 国語辞書 - goo辞書
2 or 2.2? According to 2 there has to be a potential form at the end? So is the full sentence: 罪を問うにも やつらは表向き なんら手を汚してねえ(から罪を問えない）｡ Can it be used without a potential form?

And what about 2.2? Is it limited to volitional forms? It doesn't strictly state that it must be used like this, but there's the （多く「…うにも」の形で part + the volitional example :/

I think you are talking about
1-2 （「…にも…ない」「…にも…ず」の形で、下に上の動詞の可能形を置き）その動作がどうしてもできない、または、その動作をするのをためらう、という意を表す。
and
2 《接続助詞「に」＋係助詞「も」》（多く「…うにも」の形で）逆接の仮定条件を表す。.

It's #2 an adversative conjunction; 罪を問うとしても / 問おうとしても. 多く means "mostly", i.e., "not always".

Yeah, I meant those. Just messed up the numbering

I realize that 多く doesn't mean "always" that's why I said "it doesn't strictly state", it's just that I had only seen it used with the volitional form up till now. I thought maybe there's a different entry for this particular usage or something.

Anyway, thanks for clearing that up.