Using your brain only a little bit appears to be what got you into this pickle in the first place.
It is a fact that Skinner's AA results were better than those McCallum got in TFs (and indeed prelims). This means the US would've been better, not worse off if they'd used her, based on the only data that actually matters. This isn't a complex point, as
@MaryClare explained.
Arguing that someone had more potential on a carefully curated selection of events doesn't affect that. It particularly doesn't affect it when we're talking about someone with as many unfulfilled upgrades and consistency as patchy as McCallum.
And none of this is incompatible with thinking Grace was the better choice when it came to selection. But she was a valid pick
despite Skinner having done better than her in the end. A strong argument will embrace and explain this, instead of trying to brush it under the carpet