It was pretty fun overall and not that stressful, but it could be a bit tiring. Some of us ended up getting to see an impromptu brain surgery which was super cool. Also, everyone there was super nice.
I don't think they will matter that much. It seems like not many of your CC courses are being used to fulfill the requirements for your current degree. Therefore, in a sense, the GPA for your degree is still a ~3.7 and not affected by the old grades since you aren't using those classes. I hope that kinda makes sense. That's just my reasoning though.
Besides what rising_star said, if you don't mind the travel, it could be a good opportunity to network with other people in your field. It's also nice to hear different professors' perspectives on the field/research. I always feel like I learn a lot after talking to professors.
Out of curiosity, how many interview/visits did/will you go on? And are you currently in school, working, etc.? Just want to get an idea of what is typicial.
You could try starting general then if they seem like they're expecting more details, you can get more specific. In regards to asking about their research, just say something along the lines of "I'm not to familiar with your field but could you explain a bit more about <something you thought was interesting>". Then based on their explanation you can try to see if there are parallels to your interests or at least ask some follow-up based on what they said.
Have you emailed them to check? Although I applied for EE, one of my transcripts was marked as unofficial earlier so I emailed them and they changed it.