Hi, I try to answer what I know.
1. Yes, Playa del Carmen is a safe (of couse, some basic sense is required) and convenience place, both for restaurant and souvenior. (In fact I think the souvenior in Cancun, Cozumel, Playa del Carmen are similar).
2. It's up to you to go Xel-ha or Xacret. I haven't any comment.
3. For the 'night show', I think you need to join the local tour and stay near Chichen Itaz. The distance between Chichen Itaz and Playa del Carmen is about 4 hours. There is not bus to leave at 8pm (except those by local tour).
4. For Tulum, inside the Tulum Reins, there is a beach which is beautiful. But I agree that you may not spend for 1 day. You may take the ferry to Cozumel at 6pm (and hence get more time for relax in Cozumel).
5. If you don't have dive license, you may get a 'discovery' dive tour, or get an 'Open Water' dive license at Cozumel. It is not very expensive.
Personally, if you have about 8 days, don't just spend in the beach. You may visit other cities in Yucatán Peninsula such as Campeche and Mérida. They are quite interesting spanish colonial cities.