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

We are discussing the legal aspect of the issue - how to formulate the situation in the rules: "if a player doubts." The answer is: not at all, because only the player knows whether he doubts or not.

I will repeat once again - if stepping on a cell or flying over it is an "action," then it is generally impossible to step on sand before breaking through the Guard of Magic. And this is clearly not the case (see examples from the rules). Therefore, "action" implies something else.