I do it all the time. I keep my tree very small because it's a teensy little garden, and that reduces the amount of fruit I get in one go. So I harvest what I can and chuck it in the freezer until there's enough to do something with. Never a problem. I do separate the berries from the stems first, however - I have a teensy little freezer, too.

I cannot understand why your jelly hasn't set. This is the recipe I use - it turns into a beautiful jelly, even though it's supposed to be a cordial (syrup)! I do add a squeeze of lemon juice, which perhaps explains it. In fact, I'm sure it does, because a squeeze of lemon juice will make any jam or jelly set.

Elderberry Cordial
Simmer 2.5kg fresh ripe, crushed berries with 500g sugar until the liquid has evaporated to the consistency of honey.

I can and freeze elderberries every year and I cannot imagine removing the stems after they have been frozen. It sounds to me like a horribly big mess.

Blueberries, strawberries and raspberries can all be frozen to make a jam later. I do it all the time.

RE: The unset jelly (nice syrup though?)
Well I happened to speak to my Mother in law and she explained to me that sure-jell and regular fruit pectin should not be used interchangeably- which is why the sure-jell recipe only needed 4 1/2 cups sugar per 3 cups juice and the pectin recipe called for 7 cups sugar/3 cups juice. Needless to say I did not have sure-jell but will definately have it next time. The recipe I used did contain 1/2 cup lemon juice though. The only possible problems I can think of is 1. I thought I had 6-8qt pot (the biggest dutch oven type that comes in kitchen sets?) and the recipe said add pectin and boil hard one minute, stirring constantly. The only problem? Every time it boiled hard it boiled ALL OVER (still getting it out of my burners!) So I was letting it get boiling good at just at the point of going over, picking it up off stove for a second or two. 2.I was also unsure how to keep jars hot enough as I don't own dishwasher so I put them in oven in a pan with a couple inches of water in the bottom of it- set it to 200° but when I took the jars out there was condensation in them so they weren't "dry"- does this factor in at all or is that ok? I couldn't think of any way to dry them fast enough to pour jelly in without getting burned or having it cool too fast.

The Pomona pectin would solve any, and all guesswork as to liquid and sugar needs.

Try adding crabbe apples, they are full of pectin and make they jam set well, plus elder Berry and crabbe apple jam is amazing, we have made it every year since I was a child