its unlikely to be the valve at fault, the system is driving it on, probably not fully on, but on some. The base idle does need to be adjusted by the air bypass screw under the throttle to idle at 850-900rpm when warm WITHOUT the iscv connected. When it's correct the engine speed shouldn't change when you plug the valve back in. By the sound of it though, yours jumps straight back to 1500rpm when you plug it in irrespective of the other conditions
ECT faults which make it idle high would also make it run rich, which it doesn't sound like it is.. You've adjusted the throttle sensor now which is the normal cause of high idle on 4x4 L8, so its a bit of a mystery (assuming here that you do actually have a L8 ECU and PF09 throttle sensor and 4x4 engine loom?)
If the base idle is set properly and CO settings are adjusted suitably (maybe a bit richer than the 2.5% that you have) then it's likely it will be ok in normal use without the iscv connected, with warm engine at least.