In your case, I would check out both Princess and Royal Caribbean. Royal Caribbean is a bit more family orientated and has the rock climbing walls. I do not think that any RCCL ship that does the Alaska route has the skating rinks on board. Both lines have great kid's clubs.
As for the timing of the shore excursions, remember you are heading west, and an excursion that starts at 6 AM will actually be 10 AM your local Florida time. And remember, in summer daylight lasts much longer up in Alaska. And port times are different because of the distance between ports and tidal schedules.
Both RCCL and Princess will have quiet areas for your Dad to sit down and read and relax.
As for round trip out of Seattle versus the North or Southbound 7 day cruises from Whittier/Seward to Vancouver and vice versa..........there are positives and negatives for both. For the round trip Seattle cruises airfare will be much cheaper than flying one way from Vancouver and Anchorage. The cruise will be more inside passage with calmer seas. You will see Juneau, Ketchikan and Skagway and maybe Victoria, BC, Canada and Hubbard Glacier. For the 7 day north/southbound route, you will get the more spectacular scenery.......Glacier Bay and College Fjord; you may have rougher seas crossing the Gulf of Alaska (been there, done that, got the t shirt! lol).
If price is a concern and this is your first Alaska cruise, personally I would do the round trip Seattle cruise and when you have your first taste of Alaska and you decide you just have to do it again, the next cruise I would do is the 7 day north/south bound.