Long story short, no one knows. 2 weeks from now we should all have a MUCH better idea of how to handle things. (Too bad my trip will be done by then!) So you can make some plans in the back of your mind, but keep reading here and you'll figure out the best plan of action for sure.
That said, I like theorizing.
IF fireworks are not canceled, you can see fireworks from the castle and and then go over to the Rivers of America for a standing view of Fantasmic!. Basically you get a standing spot near Frontierland for fireworks, then the moment they stop, hustle and follow the crowd over to the Rivers of America.
The only problem there is that fireworks are sometimes canceled or cut short due to winds (it took us 3 tries to see Dumbo fly in June!). So there's no guarantee you'll see them, and even less of a guarantee you'll see Dumbo and Tink. If you absolutely want to see the fireworks from the castle, then there's not a 100% chance that you will be able to see them with Fantasmic and WoC.
At this point we just don't know, but I think it would be best to devote a separate night to World of Color. You would have a very difficult time doing fireworks and WoC in the same night, since they're basically on other ends of the resort--I'm not sure you could manage it unless MAYBE you went from fireworks to an 11:30 (or whatever) show that may or may not happen, and even then I wouldn't bet on it.
If you can swing the dinner, right now I would make the reservation, and then cancel if you decide it's not for you or if it turns out the "seats" (likely standing area) are horrible. I'd much rather have that dinner reservation than not right now and there is no harm in canceling it if it turns out it's not worth it. Plus even if you are cheap like me, you can value the dinner at at least $15 per person in food alone, so you're really only "paying" about $20-$25 for each World of Color standing spot. Yes, that's pricey, but if the show's a priority and your time is limited, that might be your best option.
Your other options APPEAR to be to try to grab Fastpasses very early in the day (eating up prime Disney time), or else wait in line for what could be several hours to get a standby spot.
But again...just guessing.
Have a great trip!