I assume that the “cannot load treasure” part of the Sea Creature rules would satisfy the above and thus prevent Intermediaro from giving treasure to a friendly Sea Monster?
Correct. Intermediario’s ability doesn’t say anything about overriding cargo restrictions, so that sea creature rule still applies.
Correct. The Musketeer’s cannon is treated like any other already on the ship, and can benefit from any other cannon abilities as long as they don’t touch its range or rank.