Can fine silver chains be hallmarked 999 on end cap if a sterling clasp is used

[CJ57]

As I already said if you have to remove any part using pliers and not manually then it’s all marked the same and I think Josef is right. You can’t open a properly closed unsoldered jump ring without pliers.We are not the experts on assay though and it’s best to take advice from an assay office as we seem to be going round in circles.
You could of course make a bespoke fine silver clasp in keeping with the rest of the work. I often work clasps into my design especially if the chain is handmade