I DID IT. I had trouble seeing the pictures because my browser (IE) was interfering with logging in. Firefox didn't do that. BTW, witht the tumbler out, you can use a flat blade to operate the ignition until your new tumbler arrives.

Yes, it was tricky:
1) only the top hole/slot has a spring (90 560SEL). The bottom hole just leads to an empty slot. Trying to feel the spring that isn't there wasted a lot of time.
2) The slots don't rotate and they are at Noon and 6, so the holes line up with the slot at the 12 O'clock, vertical position. Your probe needs to align with that one location of the slot.
3) You need to find how deep the spring tab is. it is a bit deep, too deep to trust a standard length drill bit with the key head in the way. I used my long ball end small allen head wrench (1.5 to 2mm) to probe the depths of the slots to find the ramp of the spring and found where to focus the effort. That worked to educate but the ball end of the Allen didn't seem to engage the ramp well. There are two thicknesses of the wire in coat hangers. Its the thin one that slips in. The angle of the ramp is not critical , I used 50 to 60 degrees. After a few tries, I could see the wear pattern of the spring engaging the tool ramp as it should giving me confidence of future success.

4) the black protector AND the tumbler are spring loaded as a unit and both nicely pop out together on release of the spring retainer.

3) The spring is not a coiled spring but a very bowed leaf spring the width of maybe 3/16" and 5/8 long, retained at one end to spring out. The tool engages the free end of the spring at the tool end point. As the ground ramp of the tool end, slides past the spring tab, it presses the free end down into the tumbler, like a trigger.

4) Once the retaining spring is retracted down into the tumbler, the whole thing springs toward you and you just pull it the rest of the way out. Its a bit amazing that it is retained so well by just by that leaf spring.

Mine was broken internally as well as worn. The tumblers are brass and two of them were slit and operated independently by each side of the key. This made it unlikely that I could buy the very low cost (20$) Febi unit with two keys and convert it to use my original keys.

I had just purchased two new keys from the stealer at 50$ and didn't want to waste the money. I called the stealer and he cut me a deal: new custom tumbler in 2 days (Ordered Sat, pickup at dealer Tuesday, cost w/tax $68. The Febi would have cost 1/2 that with shipping, but I'd have to use a few hours to change the tumblers with no certainty they would work well. And I won't have to use two keys: one for doors and one for ignition.

Thanks for the pictures and help!