The "true form" was actually the one claire cut up, the naked woman. The one that resembled Riful was her fake 'true form'.

As for her technique, she uses both. Originally she learned flash sword (pre time skip)...after the war in the north (during the 7 year time skip) she also learned Wind Cutter. This was because wind cutter doesn't require yoki, whereas flash sword requires the user to fully awaken their arm. And during thier absence they were all trying to avoid using any yoki to avoid detection.

When she said her accuracy had increased but the power had decreased, i believe she was talking about her flash sword....as that was the technique galatea witnessed when they faced Riful/Duff. At that time, claire could do it but not control where the slashes went. Now she has better accuracy, but because she's avoiding using excessive amounts of yoki (thats my theory atleast), its power has decreased.

And lastly, no claire and priscilla haven't fought in the manga yet.