Is there any setting of the focus on the lens (disregarding the RF) that will give clearance?
No, I've tried that a few times.

Have you used it and do the RF and lens track focus properly?
No, I haven't used it yet. But it was my father's, and our family photos from the 1960s were 'acceptably sharp'.

Also, I wonder if some of the early ones didn't have the interchangeable lens feature.
I've started wondering about that myself. If I get really desperate I could always get out the Dremel and grind the cap down...