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

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.

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.