Our vote would be to stick with the same birthday weekend (19th -21st). S has already asked for those days off, but might be able to reschedule to a different weekend. Without knowing if Scott is available other weekends, it probably doesn't make sense to switch the plan yet.
If we figure out what other weekend Scott is available I will call Dick and Clark to find out if we could still make the other stops work.
Edit: If we stick with the same weekend and try to make Scott's work on Sunday, that means we'll have to shuffle the Fri/Sat stops as well. This will mean coordinating days and times with Clark and Dick. So the sooner the better on this.