I guess I don't know for sure but I would be surprised if this were the case. NG+ doesn't scale to your level afaik, it just bumps up the difficulty a set amount, which would apply to the DLC as well in NG+.
I dont think think there are invisible enemies. Unless you mean the cainhurst ghosts but they are only invisible till you get near them..... Or maybe the witches but they are really easy and also appear when near them...
But you can gain access to the DLC pretty early. However I'm not sure its recommended you do it that early.
Edit: if you mean frenzy you just hide behind rocks / use frenzy resist / sedatives.