best way to remove a thin ridge

Good morning, I am new member to this site but I have found it very helpful over the past year and only now have encountered a problem I am not sure how to handle.

The art teacher in another building and I are fusing eight 2-plate settings for a fundraiser for the new art museum for our nearby university. It has been going very well. We are using System96. We fused all 16 plates (mondrian-like pattern) face down as per instructions I found online. I used a solid shelf on thinfire. The other teacher fused four of hers on split shelves with thinfire, so the join between the shelves left a small ridge across the plate. She thought it would go away when slumping, but no it did not. So we have two slumped with a line, and two not slumped with a line.

So now what do we do? My best thought is to re-fuse the un-slumped one face down on my solid shelf (with thinfire paper of course) . She has tested grinding on a plate we'd rejected for other reasons, and it just leaves it matte, not grey and scratchy, so it would need another step. Since they are already fused, could I get away with re-fusing using two shelves, widely spaced and with my orton vent fan to aid in circulation?

However, is there a way to fix the two that are already slumped? It is a very shallow "swoop" slump. If we grind those, can I fire polish them in the slump molds? The molds have holes drilled.

At this point I'm looking at square wal mart dishes to fill in for the four missing plates.