Can I send a push notification only to guests or only to signed-in customers?

Yes. In Plugins → Push Notifications, use Send a broadcast. Enter the Title, Message and an optional Link, then choose:

• Channel: Web + App, Web only or App only
• Audience: Everyone, Signed-in customers or Guests

Before sending to everyone, you can send a test to your own customer account. To send later, use Schedule a message and pick a Send at time.