Why don't you just buy one of the adjustable ingot moulds you can then adjust it to the width of bar you need to work with.
as for carving into graphite never used it for a mould, if its a one off i use cuttlefish you cant go wrong with it.
i use graphite crucibles for casting they burn away through time.
just seems like a lot of effort to carve into graphite for it to burn away, you would be better making a master and then using cuttlefish to make your jewellery
that way you will always have the master to us anytime you need it., you can get very good detail with cuttlefish and it takes minutes to make a mould with a bit of practice.