Thanks, eggyk, great work! I already had nightmares about aberellen after my last post... 
I'd still say the root form, as you would use it to label a month, is abrell, abrill, aberell, aberill.
The -e / -en ending may be inflections, plural and / or poetic forms. Im Aprile... would still be acceptable in modern German poetry. Not that I'm 100% certain, but it would seem very weird to me to use ab(e)relle / ab(e)rellen as a standalone month label.
As Zauriek pointed out, it appears that many of the early German works were heavily influenced by French literature, so might the mid -e- 'aberell' / 'aberill' be a gemanized version of the French 'apuril'?
Also one hypothesis I came up with would be that the reason we see month name versions over long time periods is that 13-14th century literature was copied in the 15-16th century. At least this is clearly the case in most examples I provided. In this case, it would be necessary to distinguish between the creation date of the text and the creation date of the manuscript copy. Meaning - we see month names in manuscripts that the scribe would not have used in his everyday language. Neither in his region nor time. He just diligently copied an older work - and its much more ancient language. To test this hypothesis, we'd have to look into the creation dates and places of the texts themselves to see if they correlate better with certain spelling variants of month names.
I true, this would also have consequences for the VM month names. The scribe might not have used his own language or dialect typical for a certain region or time at all, but what month names he found in other manuscripts from which he copied. Considering the eclectic nature of the VM, I would not rule this out. Ofc why he wrote those month names at all is another mystery...
I'd still say the root form, as you would use it to label a month, is abrell, abrill, aberell, aberill.
The -e / -en ending may be inflections, plural and / or poetic forms. Im Aprile... would still be acceptable in modern German poetry. Not that I'm 100% certain, but it would seem very weird to me to use ab(e)relle / ab(e)rellen as a standalone month label.
As Zauriek pointed out, it appears that many of the early German works were heavily influenced by French literature, so might the mid -e- 'aberell' / 'aberill' be a gemanized version of the French 'apuril'?
Also one hypothesis I came up with would be that the reason we see month name versions over long time periods is that 13-14th century literature was copied in the 15-16th century. At least this is clearly the case in most examples I provided. In this case, it would be necessary to distinguish between the creation date of the text and the creation date of the manuscript copy. Meaning - we see month names in manuscripts that the scribe would not have used in his everyday language. Neither in his region nor time. He just diligently copied an older work - and its much more ancient language. To test this hypothesis, we'd have to look into the creation dates and places of the texts themselves to see if they correlate better with certain spelling variants of month names.
I true, this would also have consequences for the VM month names. The scribe might not have used his own language or dialect typical for a certain region or time at all, but what month names he found in other manuscripts from which he copied. Considering the eclectic nature of the VM, I would not rule this out. Ofc why he wrote those month names at all is another mystery...
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