Obviously in a competitive tournament, where there are prizes, proxies will almost always be banned, but in casual formats, like Commander, "proxies" are common, and I, personally, don't see any problems for the most part, although I don't use them
(If you're playing Planechase, proceed to the next plane.) Playtest cards aren't legal for play in any tournament format other than Mystery Booster Limited formats
Yes, proliferate is a way to level up your creatures, and has the potential to level up your creatues at non-sorcery speed
Still, Magic's long legacy as a tabletop game means that it's necessarily had to make some compromises and weird choices, even for players who arent beginners like me
Sneak plays off an old mechanic called Ninjutsu which was also seen on Ninja creatures initially