How does the UA Tunnel Fighter fighting style's reaction attack interact with the Sentinel feat's speed-reduction effect?

Question

So lets say I'm a Fighter (F), and a Creature (C) is standing to my side, within my reach (*), like this:

o o o o o
o * * * o
o * F * o
o * C * o
o o o o o 

If that creature attempts to move past me (to the left or right) and exit my reach (into the o-zone), will the reaction attack from the Tunnel Fighter fighting style (from Unearthed Arcana: Light, Dark, Underdark!) trigger as he's leaving my reach? I would guess yes, because he has to move a few inches to begin the process of leaving my reach, at which point he's "moved more than 5 feet", which triggers Tunnel Fighter's reaction.

If I'm right about that, Tunnel Fighter and the Sentinel feat (PHB, p. 169-170) have an interesting interaction that I'd like clarification on. Assuming I'm in the Tunnel Fighter stance when this happens, I can potentially take an Attack of Opportunity upon the creature as he leaves my reach and use my Reaction to attack him with Tunnel Fighter's trigger. However, Sentinel makes that AO stop the creature in its tracks.

Can I take both an Opportunity Attack and the Tunnel Fighter reaction attack in this particular situation?

If the creature starts in the bottom-right *, I assume the answer is a cut and dry "Yes", since he's clearly "moving more than 5 feet while within my reach" and won't get stopped by Sentinel until he's already moved 10 feet. The uncertainty arises when Sentinel's OA effect prevents him from taking the second 5 feet of movement.

And in the same vein, it's clear that the answer is "No" if the creature moves directly down. It has left my reach before it has moved more than 5 feet, so Tunnel Fighter's reaction attack definitely won't trigger.

Answer 1

Yes, you can make both a Sentinel opportunity attack and a Tunnel Fighter "within reach" attack.

The key point is to read each and see what resources and ordering they use.

The wording for the Tunnel Fighter fighting style says:

While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.

And the Sentinel feat (PHB, p. 169-170) reads:

When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.

Thus the Tunnel Fighter defensive stance allows opportunity attacks to occur without a reaction, and therefore allows a Sentinel attack to happen without expending your reaction.

The uncertain area can also be solved by observing that "more than 5 feet" is anything more than 5 feet, rather than just hopping into then out of reach. Taking your example, traveling from any "*" to an adjacent is just 5 feet, but anything more is more than 5 feet.

In the case of a creature and the player in the positions you outline, the Tunnel Fighter attack would resolve just before the creature leaves your reach, and then the Sentinel attack of opportunity would resolve as the creature leaves your reach, stopping it in its tracks if successful.

To further clarify: the Tunnel Fighter attack that can be taken as a reaction to an enemy's movement is not an opportunity attack, and thus the speed reduction on a successful hit from Sentinel does not apply here.