Can we actually go to the water-cutting at Getambe on the 28th or is that locals only

It is a religious ceremony, not a ticketed show. Getambe is down by the river, early, and the people who matter are the temple officials and the tuskers, not your lens. You can be present at a respectful distance if you dress like a temple visit, no shorts, no drone, no wading in for the angle. A lot of tourists sleep through it and they are not missing a carnival. If he needs to see it, a local driver who has done Perahera week before is worth more than a pin on Maps at 4am.