Congratulations on doing all that research!
Sounds like USC would be a great fit for you and sounds like you’re trying to discourage yourself from choosing it. If you present your dilemma to your parents, what do they say?
It doesn’t exactly work like what the Chair said, you can’t call USC with McGill’s costs and ask them to match 
because they’d really need to see costs for a US peer (ie., if you were admitted to Emory or CMC), not a Canadian Public. Some universities “match” peers’ offers, but only peers’ (or rivals’).
HOWEVER he gave you good advice: if you’re sure it’s your #1, you can contact them and say they’re your top choice but your parents are pushing for McGill for financial reasons, so you’re wondering whether there’s any way they (USC) can find a way to make a gesture so the choice becomes more palatable for your parents (if you were given nothing, list “scholarship+work study” to show you’re willing to be flexible…)? Or: if you were given a small scholarship, you can ask whether they could increase the scholarship by $…,000… < amount you adjust relative to the scholarship already offered, such as $1,000 for a $5,000 scholarship, $3,000 for a $10,000 scholarship…