You're over-simplifying this a bit and missing another part of the equation: AirIt is possible, depending on the type of hydraulic cartridge used in the diverter, the ports that are not used when the diverter is shifted are pretty much closed. If the FEL ports are are tightly closed by the diverter, then it will not leak down.
Solenoid controlled cartridge valves normally have less internal leakage, and some have check valves in them so they have no internal leakage, so they provide a more positive lock on the hydraulic accessory. I don't know is this diverter has this capabilities, I am just throwing out a possibility. I guess if it doesn't, you would have to shift the diverter back to the FEL and raise it routinely. :dunno:
It would be like disconnecting the FEL quick connects. If the hose fittings are actually blocked, the FEL will not bleed down.
The reason the FEL or any other cylinder attachments leak down, isn't due to the attachment itself, it is due to the internal leakage in the SCV on the tractor.
Having the solenoid controlled diverter in line after the SCV, this diverter could provide a more positively closed valve than a standard SCV. Don't know, but maybe. :dunno: