I didn't read the entire thing, but throughout Siege Bob/Void are in constant conflict, having even Norman Osborn manipulate his psyche from time to time, using like his wife or their "agreement". The ending shows a CIS-on void is still going to have a part of his psyche that he is always fighting against, even at his most powerful. Like, there are parts of Siege where Bob is considering suicide to try and stop the Void. CIS-on, neither Bob nor the Void, from what I can find, have ever been shown as "stable".
I can't find the issue, but when the shard of the void attacks Emma, she is able to contain it in her diamond form (according to comicvine). Onslaught should have Magneto level shields, and even if that isn't convincing, they are tested in the Onslaught saga, and should be enough to keep such corrupting tendrils at bay, even if you don't give the shields full "magneto" status.
anyways, my thoughts, it might go to void vs a base level onslaught, but I still think he could probably win the majority by exploiting Void's mental state.