Update on my experience with CE2's on the AVS:
The last couple of days i have been using a regular size CE2 in the AVS with no leaks.
The regular CE2 with 510-threads will fit (barely) in the AVS; the top ring and mouthpiece don't screw al the way in, but it works. I have now taken a couple of mm's off of the CE2 with a dremel, and now everything fits.
No leaking into the collector tank so far

I had one problem though: after a couple of refills, i managed to flood the CE2. Somehow, the CE2 overflowed even though the AVS was only half full!? I used a syringe to drain the CE2 while it was in the AVS, and all was good again.
I use the AVS with the liquid control ring removed; i use 50/50 VG/PG, so it's quite thick!
I think the initial problems with the XL-CE2's were due to the CE2 not sealing completely at the top.