Interesting stuff.
Lets test something:
Paladin 7 - Oath of Vengeance - Relentless Avenger: "When you hit a creature with an opportunity attack, you can move up to half your speed immediately after the attack and as part of the same reaction. This movement doesn't provoke AoO"
Would Tunnel Shooter work without the +1 to hit? (Just the ignore cover/fire in melee)?
"and as part of the same reaction" I would interpret this as a key requirement. If your OA doesn't use up your reaction then no triggering of Relentless avenger since that was intended as the limiting mechanism. Since this is effectively in playtest mode I wouldn't get too hung up on RAW.
Likewise, with Undying Light, if you actually want a living Patrons, it'd be as easy to introduce one as it would be to remove the devil from the Fiendish pact.