if they can ignore terrain for movement then they can use it while entering hindering terrain.
They could also copy a TA that allows them to ignore hindering terrain.

Cloak&Dagger cannot use Swingline - they do not meet the prerequisite.
If the Astral Plane BFC was in play though, they could enter hindering terrain and then use Ambush during the attack.

Yes (actually, that's why I forgot they normally have to stop when entering hindering terrain : I played them with this very feat assigned).

EDIT : and that wasn't even legal... argh... I'm making way too many mistakes these days.

* goes away *

Yes, they're ignoring hindering for movement, so they would not have stopped. (And since the attack is not movement, it isn't ignored for the feat.)

ok ty that help a bit

Good call.