In the preview, Tom clearly seemed to know that there might be a delayed merge. We were not shown them being told, but I think it is safe to assume they were told at some point. Perhaps at the RC when they recapped the prior TC and talked about Silas getting booted or perhaps at TC. I tend to think at RC. There is a lot we are not shown and for us that would have just been info we got already the last episode.
As far as things getting played out, remember that MB likes to keep them guessing and make things unpredictable. Post-merge, ratings dropped off for S2. The show is just more interesting and less predictable prior to the merge. Why not delay the merge one more episode and give the cross-tribal alliances 3 more days to try to solidify.
If you argue they can merge because without a merge, things are pretty predictable, I would argue that things are pretty predictable no matter what. We all know old-Samburu is soon to be extinct. We all believe Clarence will be the first to go among the old-Borans. Only the final 5 (which will be covered completely in the last 2 episodes) promises any real drama (don't we all believe we already know who the final 5 will be). So at this point, just stop watching and tune back in for the last 2 episodes (just joking AyaK--I know you would not miss a second of any episode of Survivor).
So, given that, why not make things a little more unpredictable and interesting and confusing to the players and delay the merge one episode.