From what I can tell from your pictures, the way to remove the knob is to turn the key and then push in the retaining clip through the hole in the collar.
You can't get at the retaining clip any other way. I re-key several of this type of lock a week and if the customer has lost the key, I have to pick the lock and turn the core.
In most kik locksets, the tailpiece prevents the retaining clip from being depressed. You may need to turn the knob to line up the hole and the retainer AND turn the key to allow the retainer to be pushed in AND pull the knob off at the same time. Yes, you need three hands
