Are the seeds necessary when making marmalade to extract pectin? If so, should the seeds be whole or ground up? Is the best way to keep them separate with a cheesecloth bag? This recipe is with Seville oranges and low in sugar.
Without seeing the recipe and procedure, it is hard to say if the seeds are necessary, there are many recipes for Seville orange marmalade that don't call for the seeds, and some that do. There is a good chance that they aren't essential to the recipe, that being said:
There is pectin in the seeds of citrus, but there is no need to grind or chop them for it to be released. Tying them in a cheesecloth bag, that would be easy to remove after cooking, would be fine.
(If the recipe uses added pectin, then the seeds shouldn't be needed at all.)