1 and 2 are intentional for a variety of reasons (the Banker has the option to announce tax changes or keep it quiet). 3 has significant merit, though. Even just limiting it to the flat 5% per day raise like on the taxes for duchies would make sense.
If the announcement to the realm isn't mandatory then it won't prevent anything since the banker can still raise the taxes to the max without anyone necessarily noticing.
edit: Also, hiking up the tax rate isn't really the biggest issue with the banker walking away with lots of gold. It's that he can change the distribution so pretty much all the gold goes to himself. Limiting that isn't really practical either.
