As you suspect, the one way trips are generally a bit more expensive in the long run due to the open-jaw airfare. A good TA can find an open-jaw at considerably less than two one way fares, but still probably more than a round trip.
College Fjord does get some good reviews. It did not impress me, in no way comparable to Glacier Bay.
Crossing the Gulf of Alaska to Anchorage was one of the few times we encountered really rough water in Alaska.
It is not necessary to go on to Anchorage in order to see Denali. There are many variations on cruise tours which will take you there.
Alaska cruises have so many choices available, and you are asking very basic questions, so the very best advice I can give you is find a good local Travel Agent who is a CLIA Accredited Cruise Counselor (look for the agent's diploma on the wall, not just an agency sticker). Ask your friends and co-workers for a recommendation. The service is entirely free to you. ACC's are trained to find the very best cruise for you, after learning your personal situation and preferences, and work hard to get you the very best rate.
You can check out reports and pictures from our 8 trips to Alaska, including most of the options, in the link in my signature below.
Have a GREAT cruise!