1. Shinki abilities are restricted to use when they are in their vessel form.

2. As far as I can tell from canon information, boundaries cannot be used to kill an ayakashi (aside from extreme circumstances, like when an ayakashi is being purified from the body of an offending shinki).

3. Nope~ Boundaries block only ayakashi, but can be altered to allow certain ayakashi through, which is seen when Daikoku makes a boundary to block Yukine, but Hiyori is able to walk through the boundary unharmed.