I found the memory buttons on the driver seat to be a useful backup.
Regarding the keys...On F series cars the driver's settings are assigned to a profile and each key is then assigned to the profile in use when the key is present. This permits different drivers' including a guest to use the same key without changing settings on a particular profile. On E series cars each key has a single profile.
To set up each key I opened a profile in iDrive, renamed it, then set all the settings. We repeated these steps to set up my wife's profile and key. We've had no issues with personalized settings ever since.
I agree the Owner's manual is lacking an initial setup guide.