I have no idea how I did it, but I now managed to unlock Arcanine! Didn't really do anything different from last time, though? Maybe I got a higher score?
Used Growlithe as support, caught both Talonflames with 6x Carbink, 62 combo. 62% dex completion, too, though I doubt it'd matter for someone like Arcanine.
EDIT: Also unlocked Noivern using Noibat as support, though again I have tried that before... got a 50-something combo on Hawlucha with 6x Togepi, and that did the job.
This game is so confusing...
EDIT #2: And Aurorus was also unlocked, simply by using Amaura as support and getting a 40-ish flawless super effective combo on Amaura... And again, I already tried that previously... Could these really also be affected by Pokédex completion?
EDIT #3: Also, for a completely new question... Is there any way for a Pokémon to lose their friendliness?