Undetectable semi-cheating? I bet I would play already 200 ELO better if I'd only check if my move isn't a complete tactical blunder. This obviously begs the question if engine cheating can be as easily detected if

I'd only use an engine that way
I'd constantly play only the second- or third-best move (of course there are forced moves, but it is probably unsuspicious to recapture the queen just traded...)
I only test critical decisions

After all, detection methods are meanwhile far more advanced than &quot;plays the engine move&quot;. (I once managed to play the tablebase move for 10 or so moves in an endgame, but then, I'm very good at endgames...)
By the way, the questions would also be interesting for normal computer chess.
Disclaimer: You can tell me, as I'll never played online chess in my life and never will, especially due to the cheating possibilities. :-)
Yes. You will still be detected just that the site is going to need more games. You are more likely to stay under the radar, but that doesn't mean you won't be caught. Cheat detection has never been just comparing your moves against the computer.
To understand, online cheat detection is an application of anomaly detection in data science (google &quot;Anomaly Detection&quot; if you need help). Chess.com calculates dozen of metrics such as the probability of beating a higher rated opponent due to luck, and many other metrics that they will never tell you. They are also going to do it for everybody in the database. Once they do it, they will then have a distribution of the metrics for each rating group.
If you are a smart cheater, you will win games slower than a dumb cheater but still more statistically significant than everybody else not cheating. Chess.com will create a mathematical model such as hypothesis testing to derive a p-value probability of you cheating given a null hypothesis that you aren't a cheater. As you win more and more games, the standard errors of your profile will get smaller and smaller, your p-value will also get smaller. It will just be a matter of time until chess.com bans your account.
Even if you don't end up winning, your in-game performance such as your ability to stay consistent with the top-three computer moves or your ability to play time-pressure high complexity positions while not blundering will be statistically different from everybody else in your rating group. Chess.com will be able to detect you even if your moves don't match with the computers.
That's exactly how you see why cheaters like Dewa Kipas were able to cheat for a few weeks until his match with IM Levy Rozman. (https://www.chess.com/news/view/most-watched-chess-stream-in-history-dewa-kipas). Chess.com needed time and data to build up Dewa Kipas's cheating portfolio.
You will be caught given a high enough number of games, but a single game won't be enough.