That arm #48 could possibly be out of line because although, as you noted, it has 2 screws, only the outer screw (the right screw in Fig. 6) holds it in place. The inner screw (not numbered) shown just to the right of the U-shaped spring-latch (#61 in Fig. 6) goes through a rather large hole in the arm #48.and is there for the purpose of controlling how far the arm can be pressed downward by the shutter button #49. Because of the large hole the arm #48 could possibly swing far enough one way or the other so that the small bent-down tab on the end of arm #48 doesn't properly line up with either of the holes #47 and therefore does not hold the swinging lens in place while the shutter is being cocked.
I have a beater 4C where the end of the arm #48 that is bent downward into a tab to engage the holes #47 but is bent from a small projection on the side of the arm so that it runs parallel to line #10 - #10 (in Fig. 4) rather than perpendicular to line #10 - #10 (as shown just above shaft #26 in Fig. 10). On my 4C that tab is worn and rounded so that it does not hold the lens even though it is properly seated in one of the holes #47. That could be your problem if the tab on arm #47 fits into the hole properly but pulls out it and releases the swing lens as soon as you start to cock the camera.
You might be able to fix the problem easily if the arm #48 is just not lining up properly with the holes #47 . . . that you can see by using a flashlight and looking inside to see if the Arm #48 is lying flat against the D-shaped plate #31. If it is standing away from the plate #31, then the tab has not seaten in one of the holes #47. In such a case, you might use a small dental probe to move the arm toward the front and rear of the camera to see if the tab drops into a hole #47 and the arm flattens against the plate #31.
If the arm #47 is flat against plate #31, indicating that the tab is seated in one of the holes #47, but will not hold the lens in place while cocking the camera, then either the tab or hole #47 (or both) is worn and rounded allowing the tab to slip out of the hole when any cocking tension is applied. In such a case you would have to disassemble the camera and file the tab and or holes to remove the rounded surface if only a small amout of filing is needed. If badly worn, you might even need to grind off the old worn tab, superglue or epoxy a small flat piece of metal to the end of the arm #48, bend it down, and with a Dremel Tool grind and shape a new tab. A pain in the but to do, but not too difficult a job . . . it is what I will need to do to my 4C.
I know this is a rather long message, but hope it is of some help to you.