Interesting stuff.
I just reread it and agree with you. It's pretty much 3E AoO.I don't think you get a free attack if someone approaches you. You only get one if someone move more than 5 feet within you reach (walking around you).
They make it REALLY confusing publishing two different Warlock archetypes with extremely similar names (Undying vs. Undying Light).
Here's the issue: one of the major themes/balancing factors of the warlock class is that their power comes at a price. They bargain with powerful, dangerous beings to get their powers. And eventually, somehow, they have to pay. (Or cheat.) Every GM handles this differently: it can be a personal side quest, central campaign plot line, or handled "off-screen", but it's always been part of the warlock class. You play a warlock, you're going to have "patron problems" at some point.
They've kinda thrown all that aside with the Undying Light warlock, so yeah, I'll definitely come up with an actual Named Outsider associated with positive energy (or maybe a celestial?) if I have someone play one of these in my campaign. All I can say for sure is that whoever wrote this totally failed to "get" the central theme of the warlock class. Mechanically, it's interesting. From an RP standpoint, it's a failure.
Tunnel Fighter is a bit too complicated for a Fighting Style, compared to all PHB ones. Considering that it has 2 benefits and 1 cost (the bonus action to activate), I think it could be better with just one of the benefit (e.g. the unlimited OAs) but no additional cost.
They make it REALLY confusing publishing two different Warlock archetypes with extremely similar names (Undying vs. Undying Light).