This is going to sound contrary to just plain wrong, but try it anyway, it may work and at this stage it might be worth trying. Make the rear der.cable looser than it should be when there is no tension on it.
Set the limit screw so it won't fall off of 9th. Try downshifting and use the barrel adjusters to adjust the tension. Sounds like it is pulling too much cable going from 9 to 8, so might be starting out with just a little too much cable tension? Once you do get it working, let us know how and what worked if you would.