Your logic is flawed. You say incompetence is the most probable explanation, but the most probable explanation is the one where you don't have to invent facts to fit the theory.
If it is a mistake: (a) this requires an explanation of why he even considered playing an advantage after Haaland had been fouled. He recovered his feet, yes, but was still in his own half, had few passing options available, was facing away from goal and travelling in the general direction of his own corner flag. Eight or nine times out of ten (my own broad assessment) you don't actually see any advantage arising in those situations. So his reasons for viewing that as a potential advantage are highly surprising for an elite level ref
(b) it also requires an explanation of why he appeared to signal that he was playing an advantage when, according to Webb, he didn't intend to do so. This is an elite level referee who has years of experience behind him and is judged (amongst other things) on his ability to communicate effectively. It is possible that somebody well used to using simultaneous hand gestures to indicate the reason for a decision made such a gesture without actually meaning to, but it is pretty exceptional. I cannot think of another example of any referee at elite level doing this ever. Maybe you can.
Simultaneous with the hand gesture was him not blowing his whistle at that point. Initially when the challenge was made, the ref put his whistle towards his mouth but then drew back from blowing. Whether he had decided to play the advantage at that point or whether he was waiting and seeing, the impact of that was clearly that Haaland thought he was playing on, that is obvious from his actions in getting up and playing the pass and his reaction to the whistle being blown only after he had put Grealish through on goal. Whether he says 'play on' or not, the context of his actions clearly led Haaland at least (not to mention 50,000 supporters in the stand) to believe he had either decided to play an advantage or was waiting and seeing. If he had decided to penalise the foul, these actions are inexplicable. On any view, signalling something that you didn't intend to signal is a major error.
(c) It also requires an explanation of why, if the explanation is that he was waiting to see if an advantage accrued, he didn't then wait to see if any advantage accrued. If the 'wait and see' explanation is correct, it beggars belief that he allowed play to develop while Haaland was recovering his feet but then blew up when he had done so and had executed the pass. You have said that he blew because he thought it likely Spurs would recover the ball. That is just inventing a reason. There is absolutely no evidence that this is why he blew. That suggestion is no more and no less speculation than the thought that the Spurs defenders would have caught up with Grealish, or that Grealish was offside, or whatever. You have also said he would not have wanted to bring the game back for the foul if Spurs recovered possession. Again, this is inventing excuses for why he did not wait and see whether an advantage would develop. I agree that if he doesn't commit the third error we probably don't look at the other two, but that doesn't mean they weren't errors. But if they were errors, they were absolute stinkers.
So the 'incompetence' theory involves coming up with explanations for these three major errors, each of which was committed within a very short period of time, each of which benefitted the offending team, each of which was committed by this elite level referee with years of experience behind him.
The 'incompetence' theory also requires an explanation of why, when other high profile errors see referees being 'rested' in the EPL the following week, this one is simply waved through as 'human error.' The reaction on his face is consistent with him making a mistake, but it's equally consistent with the knowledge that he reversed a decision when he should not have done so. Either way, the reaction you mention was not surprising. This is simply confirmation bias at work.
The only other plausible explanation is that he initially played an advantage (which he signalled) and then changed his mind. That is precisely what it looked like in real time, it is precisely what it looked like on the replay, and it requires zero facts to be invented to fit the theory.
Why he changed his mind is a different question.
