Could Old Testament prophets understand how the Messiah's coming kingdom and His suffering for sin fit together?
No. The Old Testament prophets had an idea of both the coming King and His kingdom and a suffering Savior who would pay sin's penalty, but they could not put the whole thing together. God did not expect them to, nor were they supposed to.
Les explains that the Old Testament prophets had an idea of all these events tied to both: the coming of the King and setting up of His Kingdom, but also that there had to be a suffering Savior to pay the penalty of sin. They couldn't put the whole thing together, and God didn't expect them to; they weren't supposed to. Even the angelic host did not even comprehend all these things. The disciples were still asking the same question about the sign of His coming and the end of the age.