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 you change the creature type of a Sliver you control 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 you control.

If Bonescythe Sliver leaves the battlefield after other Sliver creatures you control have dealt first-strike damage but before regular combat damage, those Slivers won’t deal regular combat damage (unless they still have double strike for some other reason).