How about trying a long hex head (about 4 mm or so) wrench slipped in the hole to wedge between the (no longer) captive nut and the channel?

That should "captivate" the nut again, at least temporarily, so you can remove the bolt without the nut spinning.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022