I'm trying to run at least a top 3 run for Riddle School 4 which requires essentially 2 inputs to win, the most optimal being Tab+Enter twice in extremely swift succession. If I used outside software that mapped the two keys to one on my actual keyboard, would that be considered cheating? It doesn't apply exclusively to the game so I would think not, but I just want to be sure before submitting a potentially illegal run.

Generally, mapping two simultaneous inputs to one key isn’t allowed in speedrunning. Ask in the game’s forum or discord to clarify.

Yeah what you're describing is a macro which is considered a tool-assist (i.e. usually not allowed in leaderboard runs). More broadly, using any kind of external software to accomplish something that the game itself doesn't allow is almost always banned. Always gotta check the rules for specific games to be sure, but yeah in general it's safe to assume that this sort of thing is not allowed unless something explicitly says otherwise.