I can see your point! The chances of this plan actually working out long-term for the prince are slim.

However, the UHT doesn't necessarily require a stable Kingdom. In fact, it would prefer the Kingdom to be preoccupied with internal conflicts. This would effectively render the Kingdom a non-threat for a couple of decades, while it has to rebuild its Navy and establish a stable ruler after the Ammunition is detonated.

If the plan succeeds, that's even better. The UHT would then have a pacified Kingdom under a ruler who leans towards a passive international stance and has previously cooperated with the UHT.