A battle between Darkstalker and Kirby would be an unstoppable force vs. an immovable object. Or Darkstalker would just win because Kirby can't suck him up, honestly either way.

If Kirbo were to somehow manage to swallow Darkstalker, what would his powers be?

Invincibility probably, kinda like a longer lasting version of when Kirby eats a lollipop.

Mouthful mode

Still thinking about that time when you pause the screen on Kirby Planet Robobot when Kirby has no power ups it says he has infinite power.

it depends

if Kirby is able to nullify the immortality he wins, but if not dark stalker uses animus magic and ends Kirby. if neither work, then it is probably a tie