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.

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.