I know you've been a vaper longer then I have, and unless I'm confusing you w/ someone else (and that's not out of the question, lol), you've been a Tank vaper for about as long as I have too.... so I know you know you need to leave a bit of air space in the Tank cart when filling it. I notice the bubbles you're talking about on my Tanks, but not so much on the "C". I usually have to take a few cold draws right after I refill the cart, to sort of get things moving, and I notice bubbles then..... but don't really see any when I'm vaping normally on the "C". My theory is that we see more bubbles with the regular and mega tanks because they both have a large metal mesh reservoir. So I think that fills up a bit from time to time (and that's when you see the bubbles). Sort of an irregular juice flow, dribs and drabs, sort of thing. But with the "C" there's no reservoir really, so I think there's more of a constant feed to the atty, and therefore you don't get as many noticeable bubbles (just a theory, mind you).
I don't have any leaking problems with the "C" unless I've done too many "cold draws". If I do that, then I get a bit of.....
well, more seepage than leaks really. Hope this helps, and it's nice to see you posting again.
EDIT: P.S. Good advice from Ellie (Pufferfish) too. At first I was reluctant to horse down the connection between the cone and the base (too much time being gentle with the atty-to-battery connection me thinks), but they really do need a good twist to make connection and prevent leaks.