here's one much shorter thread where I posted a video of myself explaining my grip. and how I'm choked up pretty high. Never saw you in this one, wish you had seen it at the time now. Explaining all the reasons I should change at the time probably could have helped
http://www.tabletennisdaily.co.uk/forum/showthread.php?12257-My-unorthodox-Shakehand-hold
Ahhh... maybe there was something else taking up my brain space and the problem with threads is that because of the notifications, you usually post where you last posted - it takes a lot for new topics to attract me.
The transition from service logic, I fully get. It's one of my struggles with my current grip. In fact, it is the primary reason I went back to flare blades as I found it easier to locate where on the handle I want to be with a flare.
The other stuff, I am not really sold on. Racket head speed is king in this sport. Anything that limits your ability to generate it easily is a step backwards. The main reason why your grip hurts on flat hits and drives is that your positioning your finger and thumb so high on the blade limits your ability to change the direction of your wrist motion relative to the plane of movement of your lower arm. So your finger and thumb make it harder for your wrist to wave the paddle back and forth.
I would strongly recommend that you try out the traditional shakehand grip with the handle seated in the fingers (not the palm, the fingers) with most of the pressure in the thumb and index finger and the other fingers mostly for stability. It should look from the side like what Brett is showing and it doesn't matter whether your index finger comes off the paddle or not - that is a personal choice. Mine does and I can curl it up sometimes and I really don't care. In the long term, the increased wrist flexibility and relaxation will do a lot for your game and you will learn ways to control topspin with it that you currently can't do as easily with your current grip. Because with your current grip, you mostly move the racket back and forth in one plane, while with the regular shakehands, you can actually turn your wrist around the side and over the ball to shape or avoid incoming spin. In other words, you can cheat sometimes with just your wrist doing things you currently do with your whole body. Or you can use your whole body and just let the relaxed wrist add extra speed and spin as the focal point of the whip motion in your stroke.
Since you have a good coach, she can fix all that easily if you change the grip so that she can show you how to use it. I would actually ask her to tell me what grip to use and just use that - I am paying her for a reason. It's no accident that some coaches don't coach grips they don't play with, and the biggest hurdle to improving with an unorthodox grip is that a coach cannot easily transfer knowledge about their TT to you because a lot of stuff is in their grip!