Pearlsacarolsinger
Up in the clouds
Your biomechanics instructor is also a good competitive dressage rider and does good "straight" dressage lessons. Just because you've only focused on you so far she'd still be in the mix for me, and actually high on my list.
Honestly though, none of the 3 you've listed would be my choice. I'd want an actual dressage instructor. Novice tests arent especially difficult it's only about novice/elementary dressage.
Of the 3 you have discussed, I'd say if you're happy with your jump instructor use them though
The reason RI 1 changes her tack/contradicts herself could be in response to the way the rider is riding that day, i.e. sometimes you might need more hand, sometimes you might need to be softer in the hand.
If Toby went well in your biomechanics lesson, this is the instructor I would stick with. If the rider is doing the right thing, the horse will do so too. I think you will only be able to feel for yourself what you need to ask the horse to do when you are fully in control of your own body, which is what the biomechanics instructor is teaching you.