"assume that it's not done..." Not my assumption, Ken. From the published account, whether or not any such measurement was made, it wasn't made available to the crew.
"a 50' error is within allowables.." You're getting your knickers in a twist, Ken; the RNP error you refer to (I agree 50' wouldnt be unreasonable)takes account of atmospheric and system error. But ice cap shift, if indeed there was any, would be IN ADDITION to this.