I heard the soon to come online multiplayer update will add new achievements, does this mean that the deadgod% category would have to be updated to add these new achievements, or will there be an option to use a file that auto-unlocks them like how the daily run achievements are handled. I also ask this since I noticed the rules say that co-op can't be used at all.

That's been discussed in the Discord over the past few months; please see discussion there. TLDR the category probably won't ever include the new achievements, so it would be psuedo-Dead God.