A sorcery can't be put onto the battlefield and a permanent can't transform into a sorcery. If an effect exiles Persistent Nightmare and then instructs you to return it to the battlefield, it remains face up in exile (unless that effect instructs you to put it onto the battlefield transformed, in which case it returns as Persistent Nightmare). If an effect instructs you to transform Persistent Nightmare, the instruction is ignored.

Skulk matters only as blockers are chosen. Modifying either creature's power after blockers are chosen won't cause the attacking creature to become unblocked.

If you cause a creature to have 0 power or less, use the actual value (which may be negative) to determine whether it can block or be blocked. A creature with skulk and 0 or less power most likely won't be blocked, but it won't deal combat damage and won't trigger any abilities that trigger when combat damage is dealt.