For any given Sliver, this ability can be activated numerous times in response to itself, but the end result is that only one coin will be flipped for that Sliver.

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.