Same with time compression. Edea absorbed Ultimecia's power, and that couldn't not have been possible without Time Compression and Ultimecia's defeat. And so, time compression will happen because Edea absorbing Ultimecia has already happened. The later does not exist without the former, and since the later has already happened the former must happen or the later does not exist. And since the later does exist, so does the former.
...in other words: they can't destroy the Junction Machine Ellone because those events are set in stone, which is exactly what I said =P

Granted, you don't have to have fate, or postulate that ALL of time is set in stone (you could just say that those events that have already happened are set in stone forever, but that of course leads to questions about what happens you travel back in time....alternate universe?), but what you're saying is exactly the same explanation I give, just limited in scope...