Can't seem to get enough opponents for the FFA and don't want to miss out on the cache. Can we get a way to 'try again later' rather than cancel the challenge or wait around for hours?

FFA templates will be removed from areas the next time the levels are generated. I have managed to see them fill, but it may require waiting a while. Try during peak times, but if you still can't get it it's OK to skip it.

I have skipped two of them as well after waiting a few minutes each. But yes, it was late at night when I tried. I imagine mid-day would be much quicker.

I did an arena today (Saturday 10am PST) and it filled fairly quickly. But, after winning the game isn't recognizing that I've won. It's been about 5 minutes so far and I've tried refreshing and sync'ing...

Edit: After re-reading the patch notes I see that you have to "close" the game. I didn't see any close option but I did click the blinking "back" arrow which worked.

after winning the game isn't recognizing that I've won

Please go to Help -> Contact -> Report a Bug and report it there

I finally got filled and won, but I made the mistake of continuing to play idle in another tab during the game. After winning it said I would get the reward but it synced forward and I didn't get it. Oh well.