I was in Halifax over Thanksgiving so I took a tour of Dal's architecture building, so I have some answers for you!
In regards to your first question, Dalhousie accepts about 75-85 students every year, so it's pretty competitive, but I'm not sure how they compare to other schools in this aspect. I know that they only care about your university marks (minimum B-, or 2.5 GPA), not your high school marks, so you don't need to worry so much about those (you do, however need gr. 12 math/science to get into the university math/science courses you need to have to get in). The portfolios will be more competitive since everyone applying has an extra 2+ years on any high school student, so there's that.
Secondly, your undergraduate degree gets you a Bachelors of Environmental Design Studies, followed by your MArch if you make it into/choose to go into the masters program. The program is designed for you to go into the masters program after you finish your undergrad, but you do have to have a certain grade level for you to continue on, so not everyone makes it through, but I believe most do.
I know that if you want to come to Dal just to do the masters program you have to take courses in the winter semester before and then apply. In terms of going to another school for the MArch, I don't really think that should be a problem, but I would check with admissions at other schools first.
And finally, to answer your final question, no, your first two years don't have to be at Dalhousie. They encourage you to study abroad and take a variety of courses, but you must take "a full-year course (or two half-year courses) in a mathematics or science subject that requires Grade 12 math as a prerequisite", "a full-year course (or two half-year courses) in a humanities or social science subject", and a "a half-year course that emphasizes English writing skills". The one really really important thing to remember is that your first two years CANNOT BE IN ANOTHER ARCHITECTURE PROGRAM This is because they are required to have 40% of your credits be in non-architectural courses, so the first two years is for your to get those out of the way so you can be free to just focus on architecture and design courses in your BEDS.
I hope I helped to answer some of your questions
Here's the website, http://architectureandplanning.dal.ca/architecture/prospective/, which has basically all the information you could need on this program, and if there's something missing, you can email their admissions office (the address should be there) and they should answer them.