That slotted stud you refer to is an adjuster for the glass run. It’s threaded into the glass run and doesn’t need to be removed. All you need to do is remove the nut and washer and, once all the other screws and retainers are removed, the stud will push into the door, and you can lift the entire thing out the top of the door, rotating the vent assembly 90 degrees to clear the bottom bracket as you get to the bottom of the glass run.
Why do you want to remove those two clips? They’re just for holding on the garnish molding. I don’t recall removing them on my 55 Crown, but I’ll run out and look at my 56. But I think those clips rotate 90 degrees to remove, although don’t quote me on it.
56 Meteor 4 dr hardtop
56 Crown Vic