My Greenhill 1993 edition of Wellington's Army has the footnote as I gave it. It is different from the 2nd Impression on Google but isn't clear which impression the Greenhill edition is taken from and I am not sure it makes much difference. Oman changed his mind - nothing wrong with that.
Studies in the Napoleonic Wars was a collection of papers published in 1929, but not all written in 1929. The paper on Maida for the Royal Artillery Journal, I believe, pre-dates Wellington's Army, otherwise, why mention the issue in the latter? I have no idea why the caveat was not included in Studies in the Napoleonic Wars.
I am not sure consistency is necessarily a virtue. One can be consistently wrong.
I am also as confident as I can be that the French at Maida were in columns and/or lines at one time or another.