Would it be reasonable to try to get a B.Sc. degree in Psychology, Neuroscience & Behaviour without having taken any grd 12 sciences and adv. functions? I know you can take makeup courses if you didn't take them in grd 12 but it seems like a lot of courses to take during first-year given the requirements for the degree.

It's possible, which you already seem to be aware of. Whether or not it's "reasonable" and if it's worth it for you to do are things that only you can decide.

You'll need:
-Math 1F03, and 1LS3/1M03
-Bio 1P03 and Bio 1A03 and 1M03
-Chem 1R03 Chem 1A03
-Physics 1L03/1B03/1A03
-Psych 1X03 and 1XX3

*Note the courses in red are the courses to supplement grade 12 courses.

So all in all, you have 10 courses here, which would mean a full year's work.

I didn't have calculus or physics coming into first year and I'm currently working towards a B.Sc. in PNB. I am a little behind and have to work through the summer. But if it's what you really want just work towards it. First year kind of sucks with all these dry required courses, but if you've done your research and you see that you like the PNB program, it's worth it.