Also one must sit down and interview with potential supervisors prior to entering honours years
Hah, "interview". If you want to do honours you need to have done subjects in that area. Your actual major isn't that important, but nobody is going to take you on for a biochem project if you've never done biochem before. The way it works is that the faculties release their projects and you email the supervisors and ask questions and network and meet them etc. If you haven't done the major, just tell them that but explain why you're applying and why you're interested etc.
A lot of projects will go to students doing that major for obvious reasons, but you can apply across majors as long as you've got the background. Whilst, marks are very important, the supervisor has complete discretion in choosing who they want. If you got a 99 average but there's another kid with a 75 average who has worked in that lab before...guess who's getting chosen.