If your girlfriend wants her relationship with you to last she is going to need to trust you. I obviously don't know either of you but from what you have said this sounds unlikely.
As the risk of being very blunt, this sounds like a unhealthy relationship. Unhealthy relationships at a distance have essentially no chance at all to last. The main question is probably how much and how long you and your girlfriend will suffer before one or both of you decide to give up on the relationship.
I know a few people who attended McGill. Most of them loved it. To reply to a few points that OP made:
1. Weather: It is Montreal. It is clearly *very* different weather than California (and I have lived in both places). Personally I prefer having seasons, but I can see that anything in the North would take some getting used to for a person from California. You need good boots and a good coat.
2. Academics: McGill is hard. The people that I know who loved McGill said that McGill is hard. They also said that they learned a LOT. The people that I have heard say that they hated McGill said that McGill is hard (I don't personally know anyone in this category). A's are hard to come by. The good news is that if you do well there then employers and graduate school admissions will know how difficult it was to have done well there.
3. Montrealers are rude: I have heard this from some other people who speak no French at all. In my experience if you speak French even quite badly Montrealers become significantly less rude. I have seen folks from Montreal be very helpful and very welcoming. I have gotten very good service in stores and restaurants and met many very kind people. In my experience if you go in anywhere and ask a question in French they will immediately switch to English and be very helpful. I do understand that the "switch to English" part is a hint of my ability in French.
4. Inability to find friends: This could happen anywhere. You need to get out and find friends. There is a LOT to do in Montreal and a LOT to do at McGill, but you have to do something and you need to be positive.
Regarding transferring and cost: If you are still legally a resident of California, then the California public universities will still be just as affordable as McGill. Admissions there probably does know how hard McGill is and thus IMHO it is probably worth at least trying to transfer (although it is way too late for this fall). Most other universities in Canada will be easier than McGill (Toronto being the obvious exception), and most will also be less expensive than McGill for an international student (assuming that you are not a Canadian or dual citizen). The extreme west coast of Canada (such as U.of Victoria, Simon Fraser, and UBC) will have weather that is a bit less of a shock for a California native. There are a lot of very good California public universities and a lot of other very good universities in Canada.
I do understand that a 3.21 at McGill is almost certainly significantly more difficult to pull off than a 3.6 at a community college in California. Sadly, I would be surprised if admissions at USC would fully appreciate this fact.
I would definitely call the store and offer to pay. Your friend should be able to pay by credit card over the phone. This is not a big deal for nearly all of us adults, but could be a bad situation for the teenage clerk if the item is missing and not paid for.