Thank you zxcccxz and hbmed for your encouraging words!
My MCAT is 512 (Phys/Chem: 130, CARS: 124 Massive RIP I know, Bio: 129, Psyc/Soc: 129)
Unfortunately my research hasn't produced any publications but I do have two small poster presentations that I did at my school's research days. I also highly suspect that it was indeed my ABS descriptions of activities that may have let me down. Since OMSAS didn't have much space to properly describe each entry, I just assumed it was enough to simply state what I did in each case. It was the same for UBC (where I got 25 - 50th percentile as a CV score lol). But I'm hearing people say that you have to write them nicely with a few character adjectives sprinkled here and there (e.g. I demonstrated great leadership for example). I did try harder for McGill's CV descriptions because they did give more space and guidelines to help us, so I'm just waiting for my ranking score to see how I did.
For my reference letters, my referees know me well (2 years+ and we've worked together on stuff closely) and they've all straight up told me they like me too. I can't really control what exactly they write though, nor do I have the heart to tell them what I want in the letters.
As for CASPER, I did very little prep. I did 1 - 2 practice tests from **DELETED** and that's it. I wrote 1 - 2 sentences for each question and tried my best to answer ALL questions using my own intuition, with no strategy involved whatsoever. I felt it went just OK, I didn't feel that any of my answers were outstanding or clever.