Witch form of Homura (Homulilly) was presented in the PSP game and it looks nothing like her witch form from Rebellion. One can say that this means that depending on the circumstances witch form can change. Plus, Madoka's witch form (Kriemhild Gretchen) also looks slightly different in different timelines.
Nutcracker Homulilly lacks top of the head and on the concept art for Walpurgis Night you can see that Walpurgis also does not have a top of the head. Nutcracker Homulilly also has oldee-european style dress similar to dress that Walpurgis has. Gears/clock theme that Walpurgis and Homura share, similar conical silhouette, two streak hair (braid on PSP Homililly, just streaks on Nutcracker and two-hat thing on Walpurgis), plus production notes that say that "Walpurgisnacht's and Kriemhild Gretchen's silhouettes are supposed to make a pair and they are meant to look like an hourglass". All this pretty much cements Homura as Walpurgis. Regular designs of Homura and Madoka are also make pair, with Homura being black and purple and Madoka white and pink (purple being basically pink with a lot black and gray added to it).
Also, facts aside, thematically there is no reason for Walpurgis to be anyone else beside Homura. If Walpurgis is Homura it creates dramatic irony of Homura fighting with herself and more she grows stronger the stronger her enemy becomes. And trying to save Madoka from her dark side Homura more and more corners Madoka into basically suicide. From the literary stance Homura being Walpurgis makes perfect sense, especially after "Akumura" is created. "Trying to fix everything you only make it worse and probably things would be better if you leave them alone" seems to be a theme with the series in general and especially Homura.