By rule #1, each move of a two-move turn must be individually legal for the position on the board at that moment--it does not matter whether a second move could remove any checks caused by the first move. So if there is no legal first move, it is stalemate.
I think you're saying that the first interpretation is incorrect. If there is no legal first move, it is stalemate.
Would this wording be more clear?
The game ends in stalemate if at the start of his turn a player is not in check and cannot complete either a legal two move turn or a legal single move turn.
No, it just applies De Morgan's theorem to what you originally wrote, leaving the original ambiguity intact. Since the third interpretation could be put succinctly and unambiguously as "The game ends in stalemate if at the start of his turn a player is not in check and has no legal first move," I think you favor the second interpretation, which I think is more clear when worded like this:
The game ends in stalemate if at the start of his turn a player is not in check and either has no legal move or, if he does have a legal move, cannot complete a second legal move after the first.
It seems that you favor the second interpretation, which is that the player must be able to make two legal moves to avoid stalemate. However, what if the first move is one that would normally prohibit a second move, such as a check, a promotion, a capture, or a double Pawn move subject to en passant? Would it still be checkmate in that case?
I think you're saying that the first interpretation is incorrect. If there is no legal first move, it is stalemate.
No, it just applies De Morgan's theorem to what you originally wrote, leaving the original ambiguity intact. Since the third interpretation could be put succinctly and unambiguously as "The game ends in stalemate if at the start of his turn a player is not in check and has no legal first move," I think you favor the second interpretation, which I think is more clear when worded like this:
It seems that you favor the second interpretation, which is that the player must be able to make two legal moves to avoid stalemate. However, what if the first move is one that would normally prohibit a second move, such as a check, a promotion, a capture, or a double Pawn move subject to en passant? Would it still be checkmate in that case?