So far I have stayed out of this, but now that you are starting to talk cruise I though I might pop in. Thanks Mufti for posting that link to the Top Question on the Alaska forum re cruises. Julie, that link will provide you with a LOT of information on Alaska cruises - I should know; I co-wrote it. :)
IMO there is nothing wrong with June to visit the Rockies as that is when we did a few years ago. With the restrictions of your party you would not be into hiking anyway, so it does not matter if the high trails are still closed, and the mountains are spectacular with the snow caps still in place. We were in the Rockies first week of June, but I would probably recommend mid June as the high lakes would be clear of ice by then. They were still partially iced over when we visited but I note that they were completely thawed before the end of May this year.
Are you in Rushden UK? I see that no one has yet mentioned the ability to rent a car through the UK company Canadian Affair (presently at least) with no one-way drop-off fee. I totally agree with the advice of Mufti that it is far too early to worry about air fares. You could experiment and in a few months start to monitor the air fares for 2014, or even start now to look at fares for say Aug and Sep compared to May Jun next year. One of the favourite sayings on the Air Travel forum, when the question is asked "When is the best time to buy ....", is to pay what you are comfortable with and never look at the prices again.
The same goes for cruise prices. I would HIGHLY recommend you do combine an Alaska cruise with your trip to the Rockies, which is also what we did, all self planned. You will have travelled a long way to get to western Canada, and in three weeks you DO have the time to do both. An Alaska cruise is a wonderful opportunity to visit some otherwise difficult to access regions of SE Alaska. Both Juneau (the Capital) and Ketchikan are only accessible by land or sea. The scenery and things to do from the ports are wonderful, and quite different to the Rockies, yet equally enjoyable. We did not worry about the cruise experience as such and regarded the ship to be a very comfortable floating hotel which took us to a different location each day. To experience this region you need to get out of the port towns though, and excursions can be a bit expensive.
Personally I think that you are over-planning this trip far, far too early. By all means research the regions and get a broad picture of what the party would like to do. With the assistance of contributors to these forums we can help you plan an unforgeable trip. I can assist with some questions regarding the Rockies, but others who have already posted, many of whom are on-line friends of mine, and other who may yet post in the future generally are more knowledgeable than I. When it comes to the Alaska cruise question I can provide much more assistance, as can posters on the Alaska forum where I am also a regular contributor.
To give you a taste of both experiences, these are links to my TripWOW's which are short slideshows I have created from our trip.
I hope the above is of some assistance. I now need a long rest after a lengthy squawk like a Norwegian Blue. :)