Re: Rod grip style, and Lefty's principles
Your wrist only hurts because you haven't developed the different muscles needed to do that all day. Think back to when you first started casting fly rods....your arms probably hurt as a combination of poor technique and undeveloped muscles. As you continued, that pain stopped happening because both elements improved - the same will happen if you keep practicing with your thumb on top.
Besides, when you are casting into really windy conditions and/or big flies, the extra power generated by having your thumb on top can really be an asset. Learning this technique will make you a more versatile angler.
IN regards to you having trouble keeping your line off the water on your back cast...you most likely just aren't stopping as high as you think. I used to have the same problem and the same thought that my back cast was at a high angle...it wasn't. Deliberately try to make it even higher and you will probably be surprised.