OK, you are right about the promotion, although the effect of that would (especially in Seirawan Chess) be very minimal.
You cannot get to a position with castling rights in No-Castling Chess, but you can get to a position as would be reached after castling. In orthodox Chess castling rights are not preserved forever, and usually expire when most material is still on the board. All positions that follow would also be reachable from the No-Castling initial position. But the point was really the other way around: that the No-Castling initial position could be reached from the FIDE setup. Not in the way you mention, because you could not get the Pawns back on 2nd rank, but by Nf3-Rg1-Rh1-Ng1 (repeated for the other three Rooks).
That some moves in this opening line are sub-optimal is not really relevant. In fact all opening lines but one must have sub-optimal moves in them, in Chess. The whole idea of thematic tournaments is that you want people to play from positions they would not reach voluntarily. In computer matches one often intentionally uses opening lines that are very poor for one of the players, bringing them on the edge of losing, before the engines are allowed to play the moves of their choice. And then play a pair of games with reversed colors to make it fair. This to eliminate the draw margin, which by the standards of modern engine play is so wide that virtually all games starting from an equal position end as draws. By starting on the edge the outcome can go either way (draw or loss), with about 50% chance, and a reversed pair of games has only 50% probability of ending in a draw, between equal players.
So no, poor quality of an opening line does not make a game where that opening line was played a chess variant.
OK, you are right about the promotion, although the effect of that would (especially in Seirawan Chess) be very minimal.
You cannot get to a position with castling rights in No-Castling Chess, but you can get to a position as would be reached after castling. In orthodox Chess castling rights are not preserved forever, and usually expire when most material is still on the board. All positions that follow would also be reachable from the No-Castling initial position. But the point was really the other way around: that the No-Castling initial position could be reached from the FIDE setup. Not in the way you mention, because you could not get the Pawns back on 2nd rank, but by Nf3-Rg1-Rh1-Ng1 (repeated for the other three Rooks).
That some moves in this opening line are sub-optimal is not really relevant. In fact all opening lines but one must have sub-optimal moves in them, in Chess. The whole idea of thematic tournaments is that you want people to play from positions they would not reach voluntarily. In computer matches one often intentionally uses opening lines that are very poor for one of the players, bringing them on the edge of losing, before the engines are allowed to play the moves of their choice. And then play a pair of games with reversed colors to make it fair. This to eliminate the draw margin, which by the standards of modern engine play is so wide that virtually all games starting from an equal position end as draws. By starting on the edge the outcome can go either way (draw or loss), with about 50% chance, and a reversed pair of games has only 50% probability of ending in a draw, between equal players.
So no, poor quality of an opening line does not make a game where that opening line was played a chess variant.