JohnCena20:
Hi again. Responding to your two questions separately:
Q: Is it reasonable for me to present my Canadian offer and ask USC for a faster decision (or even impact their decision, as I don't think I performed interview well at USC)?
A: No, it is no reasonable. I doubt you will have any success in getting an earlier decision from USC. They will not care that you were accepted to another program regardless of whether it's Canadian or American.
Q: Or even negotiate a better scholarship with USC (I have it from Canadian school)?
A: You might have success with this tactic. The best way to negotiate a scholarship is if you have at least one other offer in hand. You have nothing to lose by trying. At least you'll know where you stand with Marshall.