Technically the Inside Passage extends from south of Seattle all the way up the coast to Skagway. The cruise line marketing depts take some liberties, stating "Inside Passage" cruising, if they use any of these channels, even briefly.
Unfortunately, the Inside Passage is not consistent the entire length. As others have mentioned, Johnstone Strait between Vancouver Island and the Canadian mainland, is fairly narrow and very scenic. Further north in BC, the scenic Inside Passage continues with Lama Passage, Princess Royal Channel and Grenville Channel.
Sadly, the mega ships do not use the northern portion of the BC Inside Passage, as we did weekly, when I worked as Navigator for 2 seasons in Alaska, sailing out of Vancouver.
Your Seattle based ship will sail up the Pacific Ocean, only entering one of the channels to access the ports of call. Most of these channels are much wider than the Inside Passage in Canadian Waters.
Darkness will also depend on the weather, but expect it to be getting dark around 20:00 in early September. I recall we used to lose about 1/2 hr of daylight every week in September