Update - I ended up using some metal tape to add a bit of width to that pipe. That was enough to get the friction fit to work again. It took some effort to get things back together, but a hammer made things work ok. I did it by wrapping the slide-out part with 2 layers of high temp metal tape, putting the tank back on, then hitting the top of the tank with a rubber mallet (and the bottom, somewhat) until things came together. Best I can tell, the slow drip has stopped.
I could probably have also gotten a new expansion tank and the lower slide-out piece, but this solution wasn't half bad. And now I have extra metal tape!
The o-ring is kind of embedded into the tank, and I figured I'd just leave it alone and try the metal tank. I didn't really see an easy way to wedge it out like you were suggesting. I think the problem came from me just mucking around down there and damaging the o-rings in the first place, since I am pretty new to all this.