Just out of curiosity, when 2 threads are merged, are the post orders determined chronologically? Or like threat #1’s posts first, then thread #2’s? 
It’s a mixture of the two, actually. The posts from both (or all) merged threads will go in order of post made regardless of what topic it was in originally.
When two threads are merged, the new thread will retain the OP and posts from the larger of the two threads. It may not appear to be a hybrid thread at first. After the merge, more and more of the posts from the smaller of the two threads will appear until the thread tries to drag Geena Davis into the teleporter.
No, actually from the earlier, regardless of size.
As for the rest of your scenario, well, yeah.
That’s only true for the remake.
They are slotted in, in chronological order, regardless of which thread they were posted in.
Well that’s how it works on my board.