You need to think a tad more laterally re the A/S.
Not quite as simple for the new beeks to consider but you could leave space just to one side and actually move the queenless part away after a week - the ripe queen cell should be much less fragile by then. Or you could be certain of no casts by making sure there is only one cell in the broodless side - always a risk that the remaining cell might fail, but with some experience and half a dozen colonies, you could cope with that with no problems? Just a pain, and a couple of weeks lost, if it occurs!
The way I look at it is a foraging bee is a foraging bee, whichever half of the A/S it might be in. Roundabouts and swings with the total crop. Boxes are really only moved around to prevent secondary swarms with multiple queen cells.
Not much help with your initial post, I'm with Poly Hive on that score.
You could just put two supers (with frames and contents) between two queen excluders, between the top and bottom brood boxes, and not bother with the Snelgrove board at all.
The bees will equalize between the boxes, but you must remove the top box to a another site before the queen emerges, or alternatively use the queen cell frame and a couple of others with brood to make up a nuc. That would allow you to move the top box onto the bottom one and continue with double brood till you need to do it all again.
You can then use the new nuc to re-queen another hive, or even sell it for money if you have no alternative.