I recently had to experiment with this because half of my F2's didn't fizz at all. I left them for like 2 days, opened them, and there was absolutely no pop, it was like I opened a bottle of water. I added some more sugar to them and left them for another 2 days, and 2 of them did get a lot of fizz, but the rest did not. I am not sure what I did wrong here, but I can say that it works, even though half of the ones I added sugar to didn't fizz, the other half did! So it does in fact work!

Also, at that point, I didn't care to measure how much extra sugar I put in, I just poured a little bit in each bottle.

From my understanding and experience, yes. Even if you put it in the fridge but decide that you want it to ferment longer, you can. Just keep in mind that it will take it a bit to warm up to room temperature and to ferment to the point you want it to.