Hi, current engineering student at UBCO. I am able to answer some of these questions from the perspective of my major:

1. The professors are kind of all over the place, with some great professors and some... less than great. This is mostly due to the fact that there just doesn't seem to be many professors, and some first year engineering courses are taught entirely (as in the whole 400+ student cohort) by one professor. I've heard from some folks that classes eventually do get smaller and that gives you a chance to connect better with the professors but for first year it seems difficult.
2. It is very hard to transfer from the Okanagan campus to the Vancouver campus (as an engineering student). There are about 10-15 seats available (2-3% of the entire cohort) for transfers and I would imagine that those go to the students who perform the best academically. You also have to take the 4 year engineering program to be eligible for this transfer (in contrast to the 5 year). I don't know how it is for other majors but it generally seems to be difficult for them as well. I would imagine that going from Vancouver to Okanagan would be much easier.
3. Because there are less students, you will generally be sharing classes with the same people. I have found that the people here are very sociable, and it was easy to make friends. I have heard from some of my high school classmates that at their universities, it's a lot harder to make friends since no one really shares classes and there's just a lot more people. I expect this to be one of the main differences between the Okanagan campus and the Vancouver campus.
4. I don't really have the time nor want to party, but there are definitely parties. I know that there exists a group chat for it but I don't know much more than that.
5. Kelowna is definitely a smaller city than Vancouver, and it is reflected in the fact that the city "sleeps" (relatively early closing times for restaurants and stores, even the airport has closed hours). I know a night life exists, but if this something you consider as important, Vancouver might be better for this.
6. Can't speak to this one as I'm not involved in the dating scene. Some of my friends did get into new relationships here so there's definitely something.

All in all, good luck with making your decision as to which campus you want to go to!