Interesting stuff.
I'm still pretty OK with Tunnel Fighter, but to me the weird corner case isn't with Sentinel, it's with Polearm Master. TF+PM means if a single foe tries to get past you, you get up to three attacks: free OA for entering reach, TF reaction for moving more than 5 feet within reach, and another free OA for leaving reach. That's... a lot, potentially even at level 1 with variant human.
The reason I'm still pretty OK with it is that whether this actually ever happens is entirely under the control of the DM. That's a pretty determined creature to keep ignoring the Fighter. As a DM, if I have a player that wants that badly to play an aggro-drawing tank, I don't see why it hurts to oblige him/her. The opportunity cost is not insignificant.
Just talked to Mearls via Twitter.
"Nice job on the new UA. Might there be any new Invocations to accompany the new Patron?"
"If it proves popular, could get some."
So there's that.
If you can ask him if we'll ever get a non-monstrous more utility driven sorcerer -or at least one that has a familiar- you'd be my hero.
The bonus action is for the stance and this has to be spent each round if you want to have this ability active.
The way that it is written, it does not allow a character to take multiple reactions. It allows the character to take OAs without using a reaction. There is a difference.