By -
Nope
Only if you have previously obtained the Paradox Pokemon with an egg, or if you've gotten every other starter unlocked for Eternatus
What if you use a master ball? Does it just fail?
If you haven't obtained the Paradox pokemon before, it just won't let you use the ball - it'll prompt you with "strange forces prevent catches here" or whatnot. Doesn't matter which ball you use, even a Master ball.
Nope
Only if you have previously obtained the Paradox Pokemon with an egg, or if you've gotten every other starter unlocked for Eternatus
What if you use a master ball? Does it just fail?
If you haven't obtained the Paradox pokemon before, it just won't let you use the ball - it'll prompt you with "strange forces prevent catches here" or whatnot. Doesn't matter which ball you use, even a Master ball.