What a remarkable looking bird! I've never seen anything like this or pictures of anything like this! The very grey-toned mantle, head and breast are very striking and clearly not retained juvenile plumage.
I asked about the underparts because if you look on OBI there are pictures of first-winter male cumatilis with one or two pure blue feathers coming through.
I don't think this bird can be identified to species, but I certainly understand your rationale behind considering cumatilis.
Finally, thanks for the invite, but I can't make this Sunday. I will PM you though as I will be on Lamma in December.