Right that dragon would just roast the entire ship afterwards. What? You weren't seriously suggesting that a cannon would hurt a dragon were you?
Agreed. Though he almost certainly would be executed.
Unlikely. Unicorns were recognized as being pure, good, entitys who used magic, so the soldiers wouldn't be given orders to shoot. Likely given orders to capture which wouldn't work well but they wouldn't try and kill her.
If there was a meeting the Princesses would likely impress the Spanish royals with their power and glory. However I don't think the Spanish have anything to offer the ponies so I don't imagine negotiations would go much further then don't bother us and we won't create hurricanes to cripple you.