Is it a good idea to accept a culture shift from Kamchatkan to Japanese for the events?

I can't really decide if this is a good idea or not. The year is 1555 and I have Japan fully annexed. It's pretty boring event wise, mission wise and I think the shift might change that, but I don't want to do it just for fun.

Would shifting to Japanese make a -5 stability hit worth it, just for events/missions?

On a side note, when would it be worth it to reform the government? I've found no good reason to do it yet.

I don't know the Japanese events and if they're fun. But you could switch to Japanese cheaply, to mostly remove that obstacle. How? Just wait until your stability is low anyway, store up monarch points, and then set it off.

You would go from (say) 0 or -1 to -3 (so the -5 only hits you slightly) and then immediately restore stability using MP to go back to 0. This is relatively cheap and, if you go back immediately, does not trigger any bad events, as far as I'm aware.

Since you can't form Japan even if you shift culture, it's entirely useless. Events are decided by tag, not by culture, so you would not get any new events/missions/decisions.

Reforming the government will allow you to advance to better government types and westernize, so it is vital if you want to be relevant and not get wrecked by the first westerner to as much as pass by you.