The "evidence" you have to support your theories show nothing more than the parallels between Guts and the Skull Knight. Nothing more.
This is probably the evidence most damning to your case. The Skull does oppose the God Hand, but more specifically, his grudge is against Void. Now how can he have such a grudge against Void, but care nothing about Femto? Don't try to say that Void is the future Femto...
Question: How does this rivalry even begin to suggest that Guts and the Skull Knight are one in the same? It's improbable that Zodd would consider more than one person a "strong opponent"? Let's say that Guts happens to be Skull Knight. Even then, your argument here is too weak to support it.
Now you're grasping a straws. What you are trying to suggest is that this statement foreshadows your Skull Knight = Guts theory. This sort of statement is a "word from the wise" moment that is common place in many stories. Is it not common sense that someone that has experience what Casca has experience would not want to remember that ordeal? How can you misconstrue this into "the Skull Knight would only say this if he went through the same thing"?
You're right. At this point it is technically only a rumor. That still doesn't mean it's not true. But judging by the way it was conveyed to the reader, it is reasonable to assume that it is the truth or very close to it.
I absolutely hate to compare a series as great as Berserk to something like Naruto, but I feel that there is no other way to get point across to you. Think of the parallels between Guts and the Skull Knight as something like the parallels of the Sannin and Naruto, Sasuke, and Sakura. Just because they are similar does not mean they are the same.