About the fat: Make sure that, if you want to lose weight, you do it during the very first buildup periods in the late winter and early spring, when you will probably be doing long endurance rides and strenght training. Also make sure not to lose more than .5kg a week and keep a healthy diet.
Combining weight loss with high(er) intensity training usually results in poor recovery and thus, reduced performance and the risk of overtraining. Keep in mind that being lean and powerless does not get you over the mountain... a little less than "optimum" weight (in your view) and a well trained body will. I agree with you that there certainly is room for improving climbing ability with 12% body fat.
Also bear in mind that 'enough is enough': I would not focus on a specific fat percentage value to reach. The values vary a lot, depending on circumstances and measuring method, anyway, so there is no point in wanting to 'break the 7% barrier' or something like that.
There seems to be a limit of fat percentage for every person, under which the immune system is not able to keep you from staying healthy. Better be on the safe side of that limit! Headcolds prolonging themselves longer than usual or little wounds not healing quick are signs of your body not able to keep up with things...
I reccomend Joe Friels' book 'the cyclist' training bible' as a great support for designing your training buildup to the Rockies Tour! (If it is an MTB tour, check out the 'mountainbikers' training bible' version
About training climbing ability in a flat country: Check out the topic 'How do people from the Netherlands train for climbs?'