So I equipped Sabrina with a Rapier, and noticed that it gave a 'free' attack per turn as a separate attack mode... however, since Sabrina already had a full bar of abilities, this pushed the last one (Double Strike) past the end of the box, and since hotkeys only go up to 9, it could only be used by clicking it. I'm fairly sure the ability still works well, this is just a minor-ish display / convenience issue.
(aside: I also tried to Mind Control an enemy cavalry unit with Louise in this fight -- the targeting reticle said 100%, but it didn't work. I forgot to generate a log, though...)
Mind Control as an attack has a 100% of hitting (as all mental attacks do) but (as the attack description specifies) there's only a 70% chance for the Mind Control status effect to be applied.
Thanks for the report!
Bugfartboy is correct--the chance for an attack to hit and the chance for that same attack to then impart a status effect after it hits are independent of one another.