As I said, if passive immobilisation is enabled and you don't start the car within the time limit, the immobiliser is enabled again. In that case, the valet key won't start the car unless you use it to enter the EKA first.
If you locked with the fob, then use the valet key to unlock (it will only unlock the drivers door, the central locking won't work on the other doors) and try to start immediately, it still won't start the car until you enter the EKA. Once the EKA is successfully accepted, the other doors will unlock and the car will start.
However, if you lock with the valet key and unlock with the valet key and start within the time limit, it will start as locking with a valet key sets the immobiliser but unlocking with the same key turns it off again.
If passive immobilisation is turned off, once unlocked and the immobiliser is turned off (either by using the fob or entering the EKA), it stays off and doesn't re-enable after the set time so it doesn't matter how long you wait before starting the car. It doesn't need to be started within the time limit and the coil does not send a pulse so the LED on the fob doesn't flash (an easy way to see if it is enabled or not).
So, if you lock with the fob and unlock with the key, it will need the EKA.
Once that has been entered and accepted, you can subsequently lock with the key and unlock with the key and it will not need the EKA again, it only needs it the first time it is unlocked but not afterwards. I've just been outside to check on mine to confirm (which has passive immobilisation turned off) and I can lock with the fob, unlock with the key and once the EKA has been entered once, I can then lock and unlock with the key without it asking for the EKA.
If you have a faulty door latch microswitch (either the CDL or keyswitch) so can't enter the EKA with the key, on a car with a BeCM of V36 or later (mid-97 onwards) you can use a Nanocom to key in the EKA instead. If you have an early car with a BeCM earlier than V36, then when the car is immobilised a Nanocom cannot connect to the BeCM so you can't enter the EKA with it. You can only enter the EKA by turning the key in the door. That is what causes more problems than anything when someone with an early car also has a dead or dying microswitch in the door latch so it doesn't recognise the key being turned so you can't enter the EKA in the normal way.