There are MANY discussions here on where to stay in Seattle. There aren't any hotels by Pier 91. Essentially you have a choice of airport, and downtown. Arriving at 6:30, I would probably stay near the airport.
The ports enforce whatever policy the cruise line tells them. The port doesn't care. If the cruise line allows it, you are good.
So ask on the Carnival board, it depends on their policy.
Sorry...close, but not quite. First, depending on the time of day, the driving time and distance are almost the same to the two airports. Staying in downtown Oakland is cheaper for a reason...not a lot to do there.
Yes, Southwest has a big presence in OAK, but the OP is talking about flying non-stop to Toronto. Ain't going to happen out of OAK.
To add to what @Essiesmom said, all cruise ships have medical staff capable of handling an allergic reaction.
Comparing with RCI, any of what are called "mainstream" cruise lines will do well...Princess, Holland America, Celebrity (remember, Celebrity is part of RCI), etc.
They are now called "dressy nights". You may dress formally if you wish, or dressier, as you wish.
Longer cruises might be more formal, but not aware European cruises are any more dressy.
Are you asking about ship's excursions versus private excursions, or simply just walking around, (in your case, renting a van) not on ANY organized tour of any type?
People (including us) do all 3 options quite regularly.