I downgraded the game to 1.2 correctly but i dont have Void Walkers Arsenal in the bedroom, is there a way to fix this, thanks

No but in previous posts people say the downpatch is meant to give you the dlc

I think the down patch is just a few .ini files so it wouldn't really make sense that it would unlock void walker's arsenal, but I might be wrong. Last week I did the downpatch, with already having the voidwalkers arsenal installed and it worked for me.

Downpatching to 1.2 does indeed give you access to the DLC, why? no clue. I had this issue recently too and I just instead of reinstalling for the billionth time bought the dlc and downpatched then. It's fairly cheap, but if you don't wanna do that then I can't say other than try reinstalling it/removing the downpatch and trying again.