Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
This is easily a rule as intended type of thing. If you hit someone else. Sure fine, this is actually awesome and very creative and totally in the spirit of the game.
If you're just trying to break the game. Nope. RPGs aren't about trying to pull apart the systems in place for a group of friends to have fun. Not at our table anyway.
Even the damage dealt thing is silly. Is argue you cant deal damage to something that is fully damaged.
You can't "hit someone else"* because of the stipulations in contingency. RAI I'm sure they don't want you sucking the life from your left buttcheek to close up the mortal wound in your gut, but based on what they wrote, I haven't found a solid contradiction to this plan in the rules-as-written.
Also you can deal damage to something that is fully damaged. There's even a specific rule for "Damage at 0 Hit Points." under the Death Saving Throws section.
Yeah, that's not exclusionary, nor does it have any bearing on the two sentences necessary for the meme combo. In any other circumstance everyone would rightfully be calling BS on that kind of cherry-picking.
If someone were claiming:
"Well it specifically says favored enemy and you made friends with Bob's Orc character last session, so you no longer have advantage to track him."
or:
“Cavaliers also learn how to guard those in their charge from harm, […]“ You’re not the boss of this noble you’re escorting so you’re Protection reaction doesn’t work to save him.
they would be derisively ignored. And rightly so.
A statement explaining you can use the following ability for this typical use case, does not mean “THIS IS THE ONLY USE FOR THE FOLLOWING ABILITY! THIS USE AND NO OTHER!!“