My theory on why 23 should impossible
*We need a bonde, an unbonder, projection, and we also need purification, otherwise we would get 4/3 trash atoms per product, this is 15 trash atoms after the 12 th product, so we would have only 8(23-15) open hexes to output the product and we need at least 12.
*This leaves us with space for either 2 arms or 1 arm and 2 tracks
*Now we theorize on the last steps before the output,pivot/rotate seems like an bad idea(but I haven't checked all the options yet), since it gives us an weird area shape thet we need to use on both sidesof the input while not allowing us to put an arm on most places because of colissions.
*If we use translations to build the molecule we need to chose what would be the last bond, doing the one in the end of the molecule first makes an bad shape area shape thet is hard to use with 2 arms. 
*This leaves us with theconder on the bottom of the output and 4 hexes being ocupied by the molecule rigth before output that wee need to ocupy with either gluphs or tracks.