Thanks for the reply, I feel silly for not having checked the input API page more thoroughly. Would there be any method or function for simulating a player pressing a button virtually, in the manner of having a keyboard_key_press in that player object? Thanks once again!
Depends on what exactly you are trying to do - e.g. for movement you could just adjust hspeed\vspeed and clamp it to (0, maxspeed) range like the game does. For shooting weapons, there's player_fire() function. Swapping weapons is just exchanging values of "wep" and "bwep" (and a few other variables like "reload", "breload" - see fields.gml).