the Patient Pawns in Queen of the Night Chess may also move sideways. Is this the case in IX ?
I might add that the dropping mechanism in this set of variants has a certain flaw. It's possible, in normal chess, to attack a piece on the opp's first rank. In this game, assuming a similar capture happens, and that there's no way to prevent the capturing piece from 'escaping' in a sense, it can vacate the square, have the spare piece droppes, then capture a second piece. It's highly unlikely that players will allow this kinda capture, but it's still an annoyance.
There's also no retreat mechanism, maneuvering, in a chess sense, can prove difficult.
Also, doesn't that make the board particulary 'crowded' ?
As a possible solution, consider this : the piece is dropped on that square as soon as the piece originally occupying it is captured (in a circe-style.) If this square is occupied, the piece is dropped anyway, destroying any piece (enemy or friendly) that is under it. A legal possibilty is having the King on a knight's square, the knight is still on the board, it's illegal to place the knight under attack, since capturing it will destroy the king.
The pieces behind the king are replaced by a Queen capture. Pieces behind pawns are dropped as soon as the pawn leaves its file, or promotes, or is captured.