It looks well seated, and I've been on a 10 mile ride too. Also the problem doesn't seem to be the pin distance, but the fact that the pin is preventing the link from dropping into the sprocket.

I measured the 12 to 11 tooth sprocket distance against the others, and it is correct. Also because it's HG-C it's not possible to put on the wrong way round. Good call though!

Having rechecked the chain wear it's actually started to wear a little. I can *just* get the tool in on the 0.75% setting, and a ruler over 12 links shows a 1/16" stretch, so I make that 0.5%.
My worry is that even if I replace the chain, I'll still be using a link to join it!