When are you planning to do this? If it is on a weekend or close to Christmas, or if you have young children or older adults in your party, I'd say it would be virtually impossible.
If you plan to try it early in December, in themiddle of the week, I agree that it is doable (but maybe not as enjoyable as it could be since you will be rushing a bit and standing in line).
We saw lines forming on the stand by side as early as 4 pm this past December. My advice would be to show up and check things out about 3:30pm. take a look at how big the reserved sedtion is - it varies depending on how many dinner packages were sold. Then you can estimate how many stand by guests will get in. You can do other things in the area until the line gets to be near where you estimate you'll have trouble getting in. At that point you will want to get in line.
Another option is to just listen to the CP from the outdoor eating area - the beauty of the show is mostly in the music and the narration, and you can hear just fine by standing in the back or sitting at one of the tables. FWIW, some parents with young children listen from this area becasue the theater is toomuch for thier little ones.
After the show is over, about 5:45, you'll need to walk to the Studios (will be faster than taking the boat). It may take up to 15 -20 minutes to get out of the theater because the CMs block the walkway to allow all of the paticipants to cross over to the American Pavilllion (believe they wait between shows behind the Pavillion).
Allow about 20-30 minutes to walk to the Studios. If you aren't familar with the walking path, it will take longer - you may have to ask directions.
Actually, the best time to see the lights (for lower crowds) is about the same time Fantasmic starts. There is a large crowd there every evening when it first begins at 6 pm. FWIW, it it has rained in the last 2 days before you plan to go, check to see that the lights will be turned on. We learned last year that they have to wait 2 days after the end of a rain storm before they can turn the lights on as not all the lights were desinged to be for the ourtdoors. The lights were "dark" for a t least a few days last December Luck for us, we had seen it the day before the rains started.
If there is any way to do the CP and the OL on separate nights, that is the way to go.
Best wishes -