> How high level can a team get in CS (in terms of national or world rankings) without playing in a controlled environment where this sort of cheating would be more easily detected by local human observers?

Very high. You can see for yourself here[1]. Anecdotally, I had a player on my team that cheated in an (online) semi-final match. We had no idea, but were moved down a league, were disgraced, and obviously cut him. A year later, he was one of the people that toured the country with the XFX team. I couldn't believe it. Every one knew this guy cheated just a few months prior, now he was offered a contract. I was very active in the Socal LAN scene, and know at least 3-4 players that played professionally and cheated.

As far as your second point is concerned, some aimbots are so tricky, even local human observers couldn't catch them. I saw this proof of concept made by a famous cheat maker where the mouse would "tug" at the direcction of the player through a wall. The movie would be almost imperceptible by a spectator, but definitely "felt" by the player.

[1] https://www.hltv.org/blog/13538/cheating-on-professional-lev...