Q: What happens when a minion carries another minion that's carrying a minion?

Only the minions you're directly carrying count against your limit. Have fun at the carnival!

Q: If my Unit uses its extra movement to defend at a distant location, can a minion it is carrying also join the defence?

Yes! As you tap each defender, you move them to the attack's location. If you tap the carrying unit first, it can race over and bring anyone it's carrying, who will now be in range to defend.