That is a possibility, depending on what choices are made. Should the playerbase decide to push back the third portion of the event until the 2nd, we will not have that option for example, as the third portion of the event would then overlap with the timing of the intro log. We would much rather incorporate that into the event, rather than have weird timeywimey shenanigans going on when we are introducing new characters and players.

But, it does sound like you have it correct! so long as the players who tag in contribute a memory or an instance of combat, they can count the thread separately for themselves.