To remove the belt:

- remove the 13mm head nut in front of the plastic hinge that holds the upper end of the spring

- put a tire iron in the big hole on top of the plastic hinge and pivot the hinge so you can pull back on the 13mm head bolt that goes through the head and hinge

- swing the hinge the other way and pull the spring off the hinge

- pull the belt off the idler and check the idler for smoothness

- check also the tensioner on which the idler pivots for smoothness

The idler shouldn't wobble on the tensioner, the tensioner shouldn't wobble on the timing chain cover, and the tensioner shouldn't click when swung through it's range. You already mentioned that you checked that the idler is parallel to the other pulleys.

If you have to get to the tensioner, the bolt that goes through the tensioner pivot takes an 8 or 10mm hex bit. The bolt goes into an oil passage so don't be surprised to see oil. If you're lucky you can replace the tensioner without removing the fan or other pulleys. I think you can replace the idler by popping the cap and undoing the bolt.

Sixto
95 S420
87 300SDL