We have been docked while aboard a cruise ship many times over the years but honestly cannot tell you the earliest time to board since we always been on the return leg of a back to back cruise returning to Vancouver. The reason for the late departure time is that many passengers fly into Anchorage and are known to come in late via the busses. Then there are passengers arriving after land tours.
We have witnessed passengers with luggage, who we are embarking very early in the afternoon. I am of the opining with your closing documents you should be informed the earliest and latest you can board. Sorry I could not be more definitive.
It probably depends on what cruise line you are on. On our HAL cruise, we leave Seward at 8pm and I think they are saying that official boarding time will be about 5-5:30. But I also understand that often they will open for boading earlier if all goes well in the cleanup and turnaround after the previous trip passengers have gotten off. I asked the same question a month or so ago and that was the feedback I got.
We are on the May 29th Spirit as well. I read ( somewhere ) boarding would start as early as 1 PM. I'd prefer to avoid the rush of people and see some of Seward but was also unsure how close to departure you could board a ship....
Hope to see you aboard!