Mythical mon aren't required for nat dex completion. The game makers understand they are hard to get.
Not recommended, since they used hacked and cloned mons, but sure.
Yup, Switch saves are tied to accounts. So you won't be able to buy Switch cartridges with "all mons"
3DS however, saves are tied to the carts.
Hack your 3DS, then hack the mons in. Every 3DS unit is hackable.
As per my previous point, it's a lot easier when you can back up and load whatever data you want. This way, you're not beholden to others.
Anyhow, since you're gonna waste money buying hacked mons anyway, might us well use the money to buy whatever equipment you need?
To hack a 3DS? Probably a magnet if you don't have one (no seriously, any cheap fridge magnet would do), and a compatible NDS flashcart.
At your own risk, read here: https://3ds.hacks.guide/