Possibility 2) The plan was actually hatched in conversation with Mr. C after he left the Lodge at the end of s2 but before he disappeared. Cole didn't realize at that point that there were two Coopers and so on. Briggs got a sense of it after this conversation (plug in the end of Secret History here) and went into hiding without having the opportunity to directly convey information to Cole -- or he didn't know precisely what was up with Cooper at that point, just that something was off, and so on.
This is really interesting and I hadn't considered that. I would just add though that cooper had told Gordon around this point that if he should ever disappear, it's because he's trying to kill two birds with one stone. This to me gives enough evidence it was real coop who spoke about finding Judy.
I guess Mr C could have said "I'm going to kill two birds with one stone".
On the other hand the fireman asked cooper to "remember". As if to suggest "remember your original mission". Or he could just be telling him to remember this fact as it will become important.
In hindsight I have no solid facts as to why I think it wasn't mr C who said those things.
I think there are a couple of possibilities, but I first feel the need to point out that Cole does not say they had a plan to *defeat* Judy -- he simply says a plan to find her.
Possibility 1) They hatched this plan after Cooper was in the Lodge, with the Cooper in the Lodge. Briggs gained the ability to go back and forth (evidenced by his fingerprints being found 16 different times or whatever). The three of them never met together to talk about this; rather Briggs served as the go-between.
And/or there may be ways to mix those ideas differently and come up with something.
I love the thought about Briggs fingerprints popping up here and there over the years. Would be a shame to not incorporate that into the mix somehow. But I suppose the whole idea is that Briggs was onto Doppel-Cooper but didn't have the chance to inform Cole or others because of his own death. Otherwise Cole would have known about the two Cooper's (presumably) and would not have appeared so confused when Mr C surfaced.
Interestingly, after just rewatching episode 17, Jeffries says "say hello to Cole. He'll remember the official version".
To me this means that cole knows what has changed, what previously happened, and what didn't happen.
Is it possible the Judy/Briggs plan didn't happen in the original timeline, but did in the alternate one? This would account for the apparent retcon.
Or maybe it happened in a previous timeline and Cole subsequently had foreknowledge. He tells Tammy and Albert as if it happened in this playthrough, but it may have happened in the unnofficial (or official!) version.