Mate, I have had some similar issues with negatives mainly resulting from the drawing of the film over the rollers on old cameras (which you fixed on my Ricohflex, remember!) and at times on the Holga. These have resulted in white lines or scratch marks which at times have been really frustrating. I won't comment on the lith issue, more on the initial negative but I have done two things to try and sort this out.
One is a little heretical to APUG and that is to make a digital negative, scanning the neg, cleaning it up in PS and printing on Agfa Copyjet or Permajet transfer film. I have made negatives either the size of a normal negative (using it in the enlarger in exactly the same way you would the original blemished neg) or a larger negative to contact print. I have used this mainly for my 'alt' process work as the grain in the final transfer negative is large (at least on my Canon printer) and you tend to get a more pictoralist, less sharp product in the end.
The second process which I have used a couple of times (experimenting first whith negs that were not so important to me) is to use a soft graphite, HB pencil to cover the white blemish or scratch mark on the negative itself. I have heard that you can retouch both sides of the negative but I have used the non emulsion side. You have to be really careful as too hard a pencil makes it a little obvious and can damage the neg. You do need a magnifying glass and have to be a little careful but it does work. It's especially good if you have a negative that is a little grainy anyway. Another method I might try is to use graphite powder and a very fine brush (000 or 00). I use these brushes for spotting my prints.
I do know how frustrating this issue is. I have a number of really good negatives that are marked (somtimes in many places) and the straight line scratches are the worst issue. Spotting dots or threads in prints are a pain but you can do it pretty easily on B/W prints with spotting ink. These longer scratches are a bit of a nightmare really but you can sort them if you have a steady hand although it can take time.
Good luck, mate.