Can I walk in to the store without having a appointment

Yes you can; however, with all the millions of phone battery replacements going on, the chances of being able to get an appointment that way is rather slim. Best to make it in advance - I'm having to wait almost 2 weeks for an appointment for my MBP even though I was in the store for my battery replacement, but they were so busy, there was no way.

You can, yes. But it will depend on previous bookings as to whether you will be seen or not. Go first thing in the morning; that way they may be able to tell you of another time that day.