Even assuming that Purvis considers Kisume and Kasen to be "large names" out of the hundreds of touhous, and assuming he needs them to return to Gensokyo very soon, and assuming he wouldn't allow himself to use any other plot device that the one already used, I still don't see how he would punish us for entering a command consistent with the goal he has written into his story.
Here are the facts* you have recently revealed:
No helpful Kosan NPCs exist other than those already encountered.
Kisume and Kasen need to use the ship to travel back to Gensokyo, and any attempt to destroy the ship will turn out to be worse than a waste of time.
If we get captured by the soldiers, we will still get to persuade Sombraluz and Canus to not invade Gensokyo.
None of these make sense in terms of a game, a story, or a simulation. I have to ask how you know them, and why they were not made public knowledge, because they massively impact our options.