This may be a bit spoilery, so please read at your own caution.
I played NieR: Gestalt back in the day so I’ve done this before, but it’s been a while.
The criteria for route C is that I need
to have obtained all of the weapons, that means all 33 weapons, including those from the Recycled World quest (a DLC in the original game, but included in version 1.22474487139), which would show 100% completion in the weapons page. This does NOT include the free NieR: Automata DLC weapons (Virtuous Treaty/Contract/Dignity, and Cruel Oath).
Route C is different from the previous route (B) in that
you are presented a choice after the point where route B cuts off— assuming you meet the aforementioned criteria.
However, my dumb self
missed the ONE weapon in the Shadowlord’s castle; entering said castle is the point of no return for the ending.
My question is simple: is the check for whether I will be given the choice for route C resolved prior to said point of no return, or is it as long as I meet the criteria by the time the choice is presented to me? That is, is my negligence going to cause me to end with route B for a second time or will I be able to meet the criteria past this point of no return and still get the third ending?