Thanks for your very helpful advice, Stevegeo and macdonjh! I apologize if the answers to my questions seem obvious to you--I'm new to all this, and still trying to work things out.
No worries at all. It makes us feel smart...
Just to be clear, the secondary was rotating and moving freely side to side before I began working on the scope. If there were cork shims between the secondary and the corrector plate, they were long gone before I got it. No way to mark its original position.
I don't think it's that important for the secondary mirror, though I agree with bremms, centering is. Orientation is important for the corrector plate.
I am more hopeful that the corrector plate is still oriented properly. I guess I missed the threads discussing a black mark on the edge of the corrector (late-night speed reading). But the retaining ring was stuck fast--it took some pounding with a wood wedge to get it free--which made me think this might be the first time the corrector had been removed (filthy inside, cork shims still in place, etc.). So I marked the locations of the shims and serial number carefully, cleaned the corrector, and put it all back together as it had been. While I had the corrector off, I tightened the center screw in the secondary back as much as I dared, but it began to loosen again immediately after reassembly. Is the center screw all that holds the secondary in place? I thought I read that Celestron used glue to help hold the secondary in place, but can't recall where.
You shouldn't have to again now that you've gotten it free, but I don't recommend "pounding" on any part of a scope near any of the glass. In the future, a couple drops of denatured alcohol or acetone has been known to dissolve whatever it is that sticks those retaining rings and correctors. A few drops around the circumference and capillary action will wick it where it needs to go. Let it soak for five minutes or so and try again. Denatured alcohol is likely kinder to the retaining ring if it's plastic.
It's been a long time, and your scope is older than the C11 I had, but: the secondary mirror in my C11 was glued to a metal plate. The metal plate tip-tilted on a little plastic cone on the inside of the plastic housing, and was secured by the three colllimation screws. Loosening the collimation screws too much resulted in the secondary mirror falling out of the housing and into the OTA. That is not recommended. I noticed in your photo (post #1) your secondary housing has a center screw. I guess that's for securing the secondary mirror, but I've never had a Celestron scope as old as yours.
Anyway, I think I'll take it apart again, and look for any evidence of shims or markings on the secondary that would let me know how it should go. Then I'll try collimating the scope as you suggest and mark my best results. Hope not to have to bug you all again!
For initial collimation you can get surprisingly close during the daytime. Set your mount and scope up in a location you can step ten or fifteen feet in front of the scope. Tip the scope so it's pretty close to horizontal, so you can see into it while you're standing ten or fifteen feet away. Step away and look into the front of your scope. You want to be back far enough that the primary baffle tube is just hidden by the secondary mirror housing. Notice all the reflections of telescope parts you can see in your primary mirror: the secondary housing, secondary mirror, reflection of the primary baffle tube from the secondary mirror, reflection of the primary mirror from the secondary mirror... All of those reflections will be off to one "side", too. Now step back to your scope and adjust one of the collimation screws by a little bit and step back to see what effect it has. Repeating this a few time, tedious as it is, will give you a feel for which screw tips the image which direction. Keep at it until all the reflections are concentric. Really, if you stand back far enough that the primary baffle tube is hidden by the secondary mirror housing, you shouldn't see any reflections, just the secondary mirror housing and the primary mirror. The closer you step, the more reflections you can see and the more sensitive this method is (up to a point).
That should get you close enough to good collimation that your scope at least forms a decent image of a star so you can do "real" collimation with a defocused star image. It's best to do final collimation near the elevation where you do most of your observing, so I'd guess at least 45o up.