This is a very messy mission line and it roadblocks a lot of people. I can assure you there's nothing wrong with the missions, they've been around a really long time. Use a wiki guide and retrace your steps through all of the paths carefully. You'll probably find there's some NPC you didn't talk to or something along those lines. Given what you've described, it's possible you might have missed something when you tried to move on to another path.