I read best way to freeze watermelon was to cut it into bite size pieces or small strips, place them on a pan to freeze them. Then bag them up. I used my vacuum sucker/sealer machine.
I tested it out & it seemed to work pretty good but I didn't let it thaw enough so the watermelon was still bit frozen in middle. lol But it was still good to eat on hot day! I didn't want to thaw it all the way because I was afraid it would get mushy. I even cut the rind up into smaller pieces because they will eat the rind too.
You might want to see how they like it before freezing it. Just in case they don't like it. If they do love it like mine, give them a special treat of the butt end & watch the feeding frenzy.
I only tried green leaf lettuce. Iceberg lettuce is not recommended for ducks or humans because it doesn't have any or much nutritional value. I haven't tried broccoli yet. Dang I had some for dinner but didn't think to save some for ducks. I'm such a bad momma. lol
Here's video of mine devouring butt end of watermelon. Maybe because I'm new duck parent but it was fun to watch them and hear them slurping the juice. lol
That I can't answer. Mine were already at least 4 weeks old when we got them & put them on starter pellets on a schedule.
Another question I can't really answer but person we bought the ducks from had them outside already, again they were at least 4 weeks old. Depending on where you live and the weather you might need to watch for low temps.
Mine do like eating grass but really love crickets that's for sure. We have fun hunting them. Yes I participate too making it too easy for them. lol I walk around & stir up the crickets & they try to catch them. They almost always succeed.
You might not want to listen to what I do, I'm probably doing it all wrong & really spoiling them.

I'm retired so spend a LOT of time with my ducks. Probably too much but they just bring me so much joy I can't help it.
EDIT: Oops, I should have read rest of posts before replying.
Oh, I think I misunderstood. I wish mine would let me hold & pet them. Consider yourself lucky.
Mine do come to me though even when I don't have treats probably because they may think I have treat. And sometimes I can get them to follow me. Like when I take them to the good spot for cricket hunting. Or when they feel like it. lol