Am I doing something wrong? Did I destroy the "Carborundum" coating (silicon carbide) of the glass plate?
I doubt it. Silicon carbide is tough stuff: it's a 9.5 on the Mohs hardness scale, just half a notch below diamond. They make router bits and end mills and the teeth of table saw blades out of it, and people clean all of those with much stronger solvents than isopropyl alcohol. I don't know what other ingredients Creality uses in their carborundum coating or whether they might be susceptible to solvents, but I'd be surprised if the alcohol you used did any real damage.
So I cleaned the bed with IPA. The microfiber towel was yellowish afterward - so I thought that this must have been printing residues. Since then every print is kind of "welded" to the bed.
I had a similar experience, except that I cleaned with acetone instead of alcohol. My impression, too, is that the yellow stuff is some sort of residue left by PLA. If you print the same object several times in the same spot on the bed, you can begin to see a slightly shiny footprint of the object on the bed. You'll eventually notice a loss of adhesion in that spot, and the bed will feel a bit slick there. A small amount of acetone on a paper towel removes the footprint entirely, but you end up with a yellow spot on the towel that doesn't appear if you use the same procedure in a spot where there's been no printing.
I've also had object stick so strongly to the bed that they seem like they won't ever come off, and I think it's just a sign that a clean carborundum bed does its job very well. The best way to get them off, at least for me, is to apply a liberal dose of patience. I usually want to remove the part right away, but I find that if I let the bed cool down even to 50 °C, it's much easier to pop the part off.
Update: With the carborundum bed, adhesion is very sensitive to the height of the nozzle above the bed. If you have the bed set a bit too close to the nozzle, the PLA seems to really smoosh into the bed and grab on making the print hard to remove. If the bed is set lower, the PLA lines sit on top and don't smoosh in at all, and you don't get enough adhesion. There's a sweet spot in between the two, where the print sticks well enough that it takes a little effort to remove, but pops off with some coaxing. For me, using a sheet of printer paper as a gauge, it's about where I can feel the vibrations of the fan through the paper, but just barely, and the paper still slides easily between nozzle and bed. Your results may be different depending on the paper you use. Another way to tell you're on the right track is that the individual lines in the skirt should fuse together, but you want to be close to the point where they don't.