Hi Caro,
Yes, do the appt. in July! You can't do the interview and use it the same day. I think someone suggested you try that as a way to avoid a separate car trip for you. However, if since you want it in Late summer, definitely go in July. You should have it in the mail by then.
Wow! All of you guys are lucky in terms of getting up your appointments set up fairly quickly! I put in my application in April, 2014 got the online okay to sign up for the appointment time about 5 days later. The EARLIEST appointment time available here at Boston Logan was the second week of August - over 4 months later!! I was bummed because I had a trip planned in July so I didn't get to use it that time. Thankfully, the August time didn't interfere with the days I work summer school, so I booked that appointment and got in and out. Then, I got my GE card in the mail about 4 days after that.
I've gotten to use pre-check this past fall with it, but this summer will be my first chance using GE.
Have fun on your trip. :)