Usually they have WOC at 9:00 p.m., and then kick everyone out and start grad nights at 9:30 I think- but grad night is showing a start time of 9:00 p.m. this year, so who knows. They may not be planning to have it those nights.
I think your plan is good. For grad night they can get a hopper but it is about $80 more than just the California Adventure ticket, so many will stick to just the California Adventure ticket. They can also get just a party ticket, but in our experience of being there on a grad night-- there are a lot that spend the full day in the park. The high schools kids with day tickets start flooding the park in the afternoon (many teens are not morning people), so starting there at rope drop in the morning and then moving to DLR later is probably a good idea. The first 2 hours the parks are open are always the best time in the parks for getting a ton of stuff done. We rarely make EE in the morning (unless there is a new popular ride on the top of our list)- but we always get there right around regular park opening.
The WOC dessert party is a great way to see WOC. We have done it several times. It is great to have a chair, drinks and snacks, and no one standing directly in front of you blocking your view. If you can only do one of WOC or Fantasmic, I would pick WOC because there is nothing like it at WDW.
If you have to give one of the shows up, I would give up Fantasmic since you have seen it before. We do like it better than WDW's version because of the pirate ship. You can book a Fantasmic dining package and with the new rules on ADR cancellations, you don't even have to decide whether to cancel it the day before, you can cancel up to 2 hours before your reservation. Realize that with a Fantasmic dining package, you sit on the ground in the viewing area, there are no seats. If you are looking at the River Belle Terrace Fantasmic dining where you sit at the restaurant in chairs, know that the view from there is not great. I would only want to sit there if we had someone with mobility problems that couldn't do the ground in the regular dining package area. I don't know what the cancellation policy is for that specific package. If I were you, and if sitting on the ground was okay (which is a great view in the dining area), I would book the regular Fantasmic dining package for Sunday night, and then decide on Saturday after you see how much you get done that day whether you still want to see Fantasmic, or cancel and do other things instead.
We have been the exact weekend in June that you are looking at many times, we have also been on the weekends in July many times. Both are good times to go- and both have passes blocked out on the weekends which make the weekends the best days in the parks at either time.