Just look at it:
"This is the cup of my blood, the blood of the new and everlasting covenant. It will be shed for you and for all so that sins may be forgiven."
It will be shed for you and for all so that sins MAY be forgiven."
MAY BE FORGIVEN!
It is not the one word "All" it is the whole phrase which defines it.
Sins forgiven means for the saved not for the damned. I do not see a problem.
As I understand this whole thing, it is a matter of semantics. There was no word for all in Aramaiac or Hebrew. So they used the word for many.