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.

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 a creature loses double strike after assigning damage in the first strike combat damage step (due to Fury Sliver leaving the battlefield, for example), that creature won't assign damage in the normal combat damage step.