1) You can start the ATPL ground school after you have an ICAO / JAA PPL.
2) Your single engine piston class rating would be valid for 2 years. During year preceeding the expiry of class rating you'd have to fly 12 hours.
3) Impossible to answer. You have to build up hours for as long as it takes to meet the ATPL requirements. However at this point of training it should be the least of your consern
4) JAA ATPL theory, CPL course, JAA ME & IR conversion.
Good luck!
Spock