I know it is not the same as seeing a bird in the flesh, but your latter photos show features which for me are borderline Mealy/Coues Arctic. The pale rump is distinct but heavily marked and this is really indicative of mealy but maybe does not rule out Coues (having seen some photos of Coues in the hand). I would previously have said the amount of markings on the rump would rule out Coues. The pale rump area does seem to extend quite markedly up onto the back and the feathering is loose and fluffy, both features can be quite good for Coues Arctic. In the latter photos the bird has smallish bill and small beedy eye - again both good Arctic features - but these are subjective features and photos can be deceptive. The underparts are pale with no biscuit brown flanks as ground colour which Lessers usually possess (hence sort of ruling out that species), and the flank streaking is borderline for Mealy and Coues. It is fairly continuous and regulated (good for mealy), but not particularly bold and dark (good for Coues). I would go for mealy as the streaking extends low down. Can't see whether there are any undertail covert (UTC) lines, almost look absent on one photo but that is blatantly unclear in these photos. That frosty white wingbar is good for Arctic, but also ok for mealy.
I think it is mealy, but it is an interesting bird. I can't 'quite' rule out Coues. The overall paleness (whitish not just sandy-buff) of the mantle that is "apparent" on some photos interests me, but this may just be photographic effect.
This is just my humble opinion, I am no expert but have found studying them in detail this winter quite fascinating.