Yes, the shadows are there without the easel. I'm quite surprised how rare this problem seems to be. I've not been able to find anything in the vastness that is the APUG archives.
I've swapped the glass around in the carrier, but still convinced it's a reflection of the rebate - somewhere, somehow. Also, the carrier window is rectangular of course, and the shadow on the left edge of the print/negative suggests this isn't the cause.