2) The head pattern is grayer and more muted. Iliaca pretty consistently shows rufous auriculars set off by a gray supercilium and nape, and a crown with coarse rufous and gray streaking. This bird's gray bleeds smoothly (not coarsely) into the crown and into the auriculars (on the left side greater than the right, I believe).
3) The back is streaked, though this is muted and cold relative to iliaca.
4) The rear flanks are blurry and have a light brown background color to them. Some iliaca have messy rear flanks, but the background color remains white or off-white.
5) Pale wing covert tips were not very strong, though this is quite variable in iliaca based on wear etc, The photos illustrate just how different this feature can appear from image to image based on angle and lighting.
After researching zaboria (i.e. western/AK-breeding Red Fox Sparrows), they generally differ from iliaca in having more extensive gray to head and back and may be a bit duller rufous overall, but are otherwise quite similar to the iliaca we see here, complete with strongly patterned back, variably pale wing covert tips, and lack of muddy flank background color.
This bird's colder and browner tones combined with the brown background color to the flanks recalls Sooty Fox Sparrow influence and seems outside the range of what is seen in 'pure' zaboria.
I am not married to this ID, but I do think it fits best. Outside input has varied from Sooty x Red to 'altivagans' (a bit of a mystery taxon, likely Slate-colored x Red) to zaboria Red to 'no idea', highlighting the difficulty of intergrade/oddball Fox Sparrow ID on the wintering grounds.