Oh, you do have a challenging surname! First, my take on your ideas:
[name]Julia[/name] Jelly–to be brutal, your surname has a somewhat comic flavor to it. Although [name]Julia[/name] is a gorgeous name, the alliteration just reinforces the effect. So, yes, it does sound like a cartoon character.
[name]Stella[/name] Jelly–yes, too many l’s. I would avoid all names that have an ‘el’ in them. The repetition of sound is too much, and it will just invite the nickname Stelly Jelly. Or [name]Melly[/name] Jelly, [name]Elly[/name] Jelly, [name]Nelly[/name] Jelly, Belly Jelly.
[name]Tea[/name] Jelly–as others have said, regardless of the pronunciation, this looks like a grocery list
[name]Violet[/name] Jelly–and, yes, this looks like you are describing a type of jam
[name]Reese[/name] Jelly–I just don’t like the way this sounds–to abrupt.
[name]Avery[/name] Jelly–I don’t like the repetition of the ‘y’ at the end.
[name]Eloise[/name] Jelly–[name]Elly[/name] Jelly again
[name]Rebecca[/name] Jelly–I think you are on the right track here, but if I were you I’d make sure she never became [name]Becky[/name]. [name]Becca[/name]? Maybe, but I’m not crazy about it.
[name]Beatrice[/name] Jelly–My second choice, but I’m looking for something a little stronger in sound.
So, I think you need to avoid J-names, anything with an ‘el’ in it, and anything that ends in a ‘y’ (or -ie, -ee). And, just to make it harder, I think I’d also avoid K-names and Y-names, just to ward off middle school embarrassment.
What I think would sound best are names with 3 or more syllables with some hard consonant sounds–a name that will, through its sounds, sort of push itself in front and not let the surname stand in the spotlight. But at the same time, I think the name needs to be at least somewhat classic and solid. I’m afraid a too frilly, girly name ([name]Larissa[/name] Jelly) will just sound lightweight and silly.
In addition to [name]Rebecca[/name] (which I like more and more) and [name]Beatrice[/name], how about:
[name]Cassandra[/name] Jelly
[name]Erica[/name] Jelly
[name]Delilah[/name] Jelly
[name]Veronica[/name] Jelly
[name]Abigail[/name] Jelly
[name]Sabrina[/name] Jelly
[name]Catherine[/name] Jelly
[name]Susannah[/name] Jelly