[RPG] Is a character with the Polearm Master and Sentinel feats unbeatable in a duel

Yes, your reach with the bonus attack is the same as your reach with the regular attack. The Reach weapon property states that:

This weapon adds 5 feet to your reach when you attack with it, as well as when determining your reach for opportunity attacks with it (see chapter 9).

So whenever you make an attack with a Reach weapon, your reach is 5 feet more than usual. There's no restrictions placed on how you get that attack. As long as you're using the right weapon, you get the extra reach.

In this specific case, you're attacking with the opposite end of the weapon, but it's still the same weapon. Your reach will be 10ft if it's a Reach weapon, or 5ft otherwise.

You are correct.

Tunnel Fighter lets you make opportunity attacks for free, and Polearm Master lets you do so at a distance.

An intelligent or reasonably aware force would have a good chance to figure this out after you skewer the first few.

As for "too good," it's powerful but not unrealistic. Choke points are good strategy in the real world, too; see the Battle of Thermopylae or various castle designs that involve narrow gates or low doorways. Given that the PC has spent a precious feat and their fighting style choice on this, they deserve to have it be pretty useful.