Wherefore art thou, aberil?

[Koen G]

I guess that a possible line of investigation could be examining whole sequences of the twelve months and rank them according to some quantitative measure...

This is probably the way to go. It would be interesting to learn what we should expect in a full series of month names that contain some of the forms. For example, in a full calendar that uses "octembre", what is given for the other months? Is there a region where we can include more peculiarities?

There must be some kind of heuristic method akin to searching transcribed legal documents that can help us with this, but I haven't found it yet.