I was able to use "De-fusion" to bring back a "Proto-Cyber Dragon" after using it as material for "Cyber Twin Dragon", which I thought was not possible, or at least on "Over the Nexus" it isn't. I'm not really sure of the ruling for it in real life (especially since this is an old one), but I thought I'd post this just in case it was a bug.
Proto was used as a fusion material, so I can't see why you shouldn't be able to summon it with De-Fusion. There's also this ruling on De-Fusion's wiki page:

"Suppose the name of "Elemental HERO Prisma" is changed to "Elemental HERO Neos" by its effect, and then it and a WATER monster are used as Fusion Materials to Fusion Summon "Elemental HERO Absolute Zero". Afterwards, if "Elemental HERO Absolute Zero" is targeted by "De-Fusion", then you can Special Summon that "Elemental HERO Prisma" which was used as a Fusion Material, even though the name of "Elemental HERO Prisma" returned to its original name when it was sent to the Graveyard."

Seems like a similar enough situation that it should apply..
Yeah I saw that , but, Prisma is able be used as Fusion Material for Absolute Zero regardless of changing its name to Neos, because Prisma is an Elemental Hero anyway. I know that Fusion Substitutes can always be Summoned by "De-Fusion" but name changes are different. "Proto-Cyber Dragon" was ruled not to be allowed to be Summoned through "De-Fusion" in the past (UDE rulings), so perhaps this rule no longer applies, yet I haven't found anything updated on the subject. But until then I guess I can accept how it is ruled in Ygopro, especially since most people would use Zwei over Proto.