I don't know how "set in stone" the absence of, or the inclusion of, streaks to the UTC's are regarding definitive ID of the said races regarding this taxa. The OP had singled this bird out as distinctively different from rest of group which were given as Common Redpolls, clearly a much paler bird than the rest, thus perhaps grounds for candidacy?
This taxa, as you know is a "bit of a minefield", hence the call to lump along with other taxa that are not properly understood, in my view if you have a seemingly credible candidate that appears to favour one taxon over the other (albeit might in certain quarters be considered marginally so), then one is at liberty to respond accordingly as one sees fit.
With respect Ken, although it might have stood out within the flock, extreme pale Meally Redpolls do exist-and can in fact be more striking than this individual.
Without relying just on the undertail coverts, the following
exilipes features (from Nils Van Duivendijk's book) don't fit this bird very well:
Ear coverts almost concolorous with rest of rear head (ear coverts normally darker in Common) You could argue that they are the same colour here, but there is also a well-defined border which might be within range for extreme Arctic?
Pale rear scaps (lacking in this bird)
Bill not especially short (nor indeed 'buried’ within facial feathering)
Upperparts with yellow-white to white ground colour (this bird has a brown ground colour to my eyes. I can’t detect any yellow tones)
Often pale hind-neck (It looks quite dark like the mantle to me)
Often broad head and thick neck (this bird is not typically broad and full-necked as many typical
exilipes)
On the undertail coverts of Coue’s Arctic/
exilipes (from Martin Garner’s Winter Challenge book):
'Undertail coverts vary from un-marked white to containing one thin slate-coloured streak. Rarely two streaks or single broader eliptical streak and a number of very thin pencil’ lines.'
Given that the subject bird has 2 broader streaks and at least 2 pencil streaks, coupled with the other features that don't fit, I think it would be misleading to suggest that this is a good Arctic candidate. It might be an 'extreme outside chance' candidate but probably safer to err on the side of caution without better evidence.