I tried to respond to your post, it is a plate not a rubber .
Any thoughts on what mechanics might change in walk-thorughs and long toss that we could try and translate to the rubber?
Walk-throughs and long toss force the pitcher to use her legs. Your DD is probably losing some leg drive and may be "guiding" her pitches when throwing off a rubber. Practice throwing off a rubber into a tarp @ 20' away and concentrate on leg drive and arm whip. Throwing into a tarp allows the pitcher to focus on her mechanics instead of the results of the pitch.