First - Depends on which bar, time of day, # of passengers, etc. We've never had a problem.
2) Only the main theatre show(s) take reservations pre-cruise. Others you sign up on board. All will have a standby line. Some are first come/first served.
3) You can reserve MDR's, but walking up usually doesn't include a long wait. Maybe 20-minutes or so max. Again, depends on time of day and number of passengers.
4) Any of them
And no, the negativity is not to be believed.