Abilities and layers, redux

I have touched this subject before, but here's a much simpler case:

1. You control a Bonescythe Sliver, and someone casts Snakeform on it. Will your other slivers have double strike?
2. You control a Stormtide Leviathan, and someone casts Snakeform on it. Will all lands be Islands in addition to their other types?

Instinctively the answer to both questions would be "no", but of course it's not that simple.

In the first case, even though the Sliver's ability has an earlier timestamp than the ability-removing effect created by Snakeform (which normally would mean that all other slivers get first strike before the Snakeform effect removes Bonescythe's abilities), these are dependent effects. In other words, both continuous effects exist on the same layer (the "ability adding/removing" layer, ie. layer 6), and the existence of one depends on the other. This makes them dependent, and for this reason Snakeform's effect is always applied before Bonescythe's, regardless of their relative timestamps. Which means that no slivers get first strike. This makes it intuitive even without knowing about layers, timestamps and dependent effects.

What is much less intuitive is what happens in the second case. One could easily think that it works the same way: Snakeform removes the ability, and thus lands are not Islands in addition to their other types. However, it doesn't work like that.

The Leviathan's effect is not an ability-adding effect, but a type-changing effect. Types are set in a lower layer (more precisely layer 4). This means that the effect is evaluated before any ability-removing effect.

These two effects are not dependent because they are on different layers. Only effects on the same layer can be dependent.

This means that even though Leviathan ends up not having any abilities, all lands will still be Islands in addition to their other types, as unintuitive as that may be.