Is there a complete different meaning behind では ？
I'd rather think では doesn't have much meaning compared to こと. ではない apparently doesn't have specific meaning for unnecessity and I guess it cannot be used with 行く from the grammatical point of view.
Let me explain in depth.
行くことはありません is separated as 行く + こと + は + ありません. こと we're using here is called "dummy noun" (形式名詞 in Japanese). So, basically, it points to 行く and is used to form a noun with 行く (to go). は is just a postpositional particle. Don't care about it too much. ありません is the negative form of ある. I don't know why, but こと seems to gain a meaning for unnecessity when it's used with the negative form of verbs.
Next, ではない. When we want to negate the meaning of 行く, we'd say 行かない because 行く conjugates along カ行五段活用. There's no room for putting ではない right after 行く. It's just grammatically wrong.
So, you might wonder what ではない is. I guess it's separated as で + は + ない, not では + ない. で is part of the conjunctive form of verbs (e.g. 好きで + は + ない). は here is a postpositional particle and ない just negates verbs.
In conclusion, Xではない is the negative form of verbs and it doesn't have additional meanings other than negation. Also, putting ではない right after 行く is grammatically wrong.