There are two separate reading/writing mechanisms in a DVD writer, one for CDs and the other for DVDs. From your description of problem 1, it sounds like the CD mechanism is not working anymore. The DVD one is still fine though. Unfortunately, if this is correct, there is no solution for this AFAIK, apart from professional repair. As your warranty has expired, I don't think there is anything that can be done about that problem.
As for the second problem, a "focus or tracking error" message means the head of the drive cannot move to the right track or position it needs to get to in order to burn at a certain part of the drive. This is typically due to the fact that the grease on some part of the rail that the head mechanism slides on has dried up. From your description it is probably one of the ends of the rail that has gone greaseless. That is why you get that error when you burn up to 90% of the disc : it is actually unable to focus on the right track position in order to burn there.
About the only thing you can do is to regrease the drive rails. It isn't really that hard at all. The most complicated part is opening up the drive. After that it is really just a matter of applying lubrication oil with a q-tip. If you have problems doing it, would pictures help? I've done a regreasing job before on my LG writer and I took pictures. The model is different (4163B) but the layout is about the same. I can post those pictures here if will make things clearer for you.