What kind(s) of adaptor plug and voltage converter will I need for the Bellevue Palace and for the MS Inspire?
You only need a converter for anything that isn't dual voltage. Most modern electronics like phones, tablets, ereaders, etc just need an adapter if anything.
The ship has both us and European style plugs. They may also have USB outlets. They've been adding these to their staterooms.
The hotel website says the desk has international outlets. I've been in several European hotels that have desks like that with sometimes US outlets and/or USB.
I would recommend bringing a European style 2 round pin adapter just for convenience. Sometimes the most convenient outlet location wise is a European plug.
If you're thinking of bringing something other than the list above (heated hair appliances, cpap, etc) that's a whole other discussion.