Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn't provide any additional benefit.
If the creature type of a Sliver changes so it's no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to another Sliver may become lethal if Watcher Sliver leaves the battlefield during that turn.