Lenses are lenses. Depth of Field/Focus is not directly related to "Flatness of Field".
Using a smaller f/stop *will* increase Depth of Field/ Focus.
Try tilting the easel in enlarging to correct perspective. Stopping down is virtually necessary.
"Apo" is a shortening of "Apochromatic" ... supposedly additional care is taken to reduce chromatic abberation - a.k.a. "color fringing"" - But I don't know --- Certainly some of the non-Apos are very good as well. Possibly, "APO=Higher Price".
Yeh, yeh... "Best" aperture is usually designed to be in the center of the aperture range .... but that is probably as over-critical as you can get. I haven't yet seen an enlarging lens that was materially - or even noticeably worse at the extremes of the range.
If they were as "bad" as some believe, they would not be fitted with adjustable f/stops.