Yes, you can get a ticket for general admission, not sure if a guided tour would be available on spot.
Yes but you will have to queue for a bit.
Yes! Even with a guide you wait whilst they pop over to the ticket office to buy tickets (albeit a much shorter line than the general public).
If you have any teenagers make sure that you have photo ID for them with their birthdate (even a photo copy of their passport). Throughout Italy and Greece we found the age of our 14 and 16 year olds questioned as it is often free or a significantly reduced price for those under 18 year olds.
Get an answer from Skip-the-line half-day Private Tour of Ancient Pompeii highlights w Local Guide staff and past guests.