I think I figured out what causes the TAC infinite. I've been able to do a TAC infinite with Sentinel, but not always, and there is one thing I've noticed that happens when I manage to do the infinite: I let the character do the TAC spin animation longer. This would actually explain Trish's, because she does an airdash down into Round Trip/delay dive kick, allowing them to spin longer. I've tried to do her infinite by hitting them the moment I come in, and it doesn't work. You have to delay it by quite a bit.
Use this:
http://www.mmcafe.com/nico.html
And it actually seems to support my idea, since he starts it off a Hyper Grav that takes a while to actually connect.