Hopefully shouldn't be an issue - too hard to draw on the computer but I'm reasonably certain that I'll be able to have pretty smooth lines for all transitions. I'll use right angle connectors to 'step out' the middle drain ports on the SDs, then have the 2x 25mm drops connect at the bottom - ideally using 2x 45º on the 'far' line, but otherwise with a 90 if I can't fit it. SD1 will have a short (~1m) horizontal run of 40mm straight into the 'back' port of the reducer wye. SD2 will have around the same length run and come into the side of the reducer wye (both via bushes).
No worries I'll give them a call tomorrow and try and make that change.
I was also thinking - the standard more common solenoid valves that have min working pressure of 20kPa -- if my calculations are correct that is only 2m of water head pressure. As long as the middle-port drops from the SDs were full of water they should reach the minimum working pressure with no worries.