Mashing is rarely a concern in fighting game cheats, it's either inhuman reactions (hard to distinguish from a lucky guess) or always guessing right on plus/oki situations.
More complex cheats don't activate every time; it's basically impossible to measure whether your opponent in Street Fighter has a "auto perfect parry 15% of the time" cheat running.
That's correct. The entire archive is only about 12GB; it couldn't possibly contain the full games.