I checked further and only one jar came out as " grape molasses." So I made a batch of vanilla ice-cream and when it finished I made grape swirls throughout it with the one jar and it is the best grape swirl ice-cream imaginable.
Unless they're terribly overripe grapes have plenty of natural pectin and don't need the commercial kind.
Jelly/marmalade can take up to 6 weeks on the shelf to set, so an unset jar in a batch isn't necessarily an indication that it won't eventually firm up.
You can re-cook the jelly to bring it to the set point but there's a risk the jelly will end up tasting caramelized and less fresh, so really your method of swirling it through ice cream is by far the better option.
I had the same problem making mulberry jelly.
But I think mine didn't get as thick because I wouldn't use all the sugar that it called for. Way too much sugar.
So instead we have mulberry syrup for waffles and pancakes ^_^
This is going to sound weird or counter-intuitive, but jams with commercial pectin have much more sugar than traditional jams without it.
It's like a self-perpetuating cycle. The commercial pectin helps set the jam but needs more sugar to work.
Ironically, unless you're looking at a reduced-sugar pectin or something like Pomona pectin (which could jell water), you will have better luck reducing sugar in a jam that doesn't use the commercial sort.