Checking the Roll Calls, here:
https://boards.cruisecritic.com/forum/233-volendam-roll-calls/
I don't see any for your cruise. But that would be the place to meet others on your cruise.
London is a large city...the sites are spread all over a wide area. However, the subway (Tube") is excellent.
As to hotels, Conrad St James is in a great location, and is an excellent hotel. London is not cheap, particularly the downtown locations.
The better place to ask this is on the Celebrity board, here:
https://boards.cruisecritic.com/forum/16-celebrity-cruises/
I am sure there will be dozens of dress code threads there. I expect this will be moved over there.
Will you have problems abiding by the dress code? What kind of sandals? Flip flops? Probably not allowed.
The roads to Ka'anapali are open. You can drive to points north of Lahaina. There are open resorts north of there up through Kahana and Kapalua. The road has to be open to access them.
When you book under HIA, both occupants of the cabin have the Wifi plan. Its for one device per person, but if you need more than one per person, just sign off of one, and use the other.
Your take on the gratuities is correct.
The letter DOESN'T say they crew made a wrong turn. It says, in essence, the home office plotted the Inside Passage (but apparently didn't book the pilot). The crew plotted the Pacific route west of Vancouver.
The offer is pretty generous. 10% of a 15 or 20 day cruise? For missing the Inside Passage? Pretty generous.
No question. Just a very brief report. There are many people who ask here about ground transport ideas and costs. Of course, most don't look for other's reports.