Could be, but it seems like talking to himself.
I think [y] = ke[y]stone is a stretch too, as in the ke[y]stone message, the [y] was part of the word "pray" taking all the [] letters and concatenating them.
Back to the "yes" possibility, maybe a synonym for "go." as in "fifth part of Stage_5, go"
Applying the same thought, that the colon separate two elements, the first element is "Stage_5", the second is "5[y]"
Rather than Stage_5 having mulitple parts (although the castle designations might be parts of the "Stage_5 campaign"), "Stage_5:5[y]" maybe "Stage_5 ... GO"
Just tossing out different ways to view the strings and messages. I don't feel strongly drawn to any particular interpretation just yet, especially not the ones that apper to be highly coded.