So does this mean that the "Mis" is not necessary?
Easiest to stick with mis and the name of the month, because the words for names of the months may have other meanings, too.
For example, medi (harvest), hydref (autumn), gorffennaf (I will finish).
Since this can mean 'Autumn and September' would anyone really say it that way?
It should read Mis Hydref.... There was a typo which allowed it sometimes to appear without the Mis - now corrected.
How important is the capitalisation on these?
Just as important as in English.
The mis is not capitalised unless at the start of a sentence, but the second word is capitalised.
Thank you for the clarification. I was capitalizing both.