your questions regarding the game rules (improving the interpretation).

He wasn't sure, but that doesn't mean he's right.

We are discussing the legal aspect of the issue - how to formulate the situation "if a player doubts" in the rules. The answer is: in no way, because only the player knows whether he doubts or not.

What is meant by "action" is not specified in the rules; for you, it's visiting, collecting, or probing, but for me, it might be reconnaissance. Also, stepping on a certain cell or flying over it is also an action for me.

I'll repeat once more - if stepping on a cell or flying over it is an "action," then you can't step on sand until you've probed it. But that's clearly not the case (see examples from the rules). Therefore, "action" implies something else.