Hello,
This is a tough one. I have re-read you post a few times now. You certainly did everything right in trying to keep your transmission maintained well.
My gut tells me that this is a transmission problem with the shuddering. You know what I would want to do is to drop the transmission pan and see how much metal is on the bottom of the pan at the magnets. Just to get an idea of how worn the transmission is. As this is the original transmission, it is even more likely that the cause is the transmission. I agree that you are right on suspecting the torque converter as the main cause of the failure. I have found the internal seal in the torque converter fails which leads to the transmission failure. The way I found it was to drain all the fluid from the torque converter and look deep inside the center hole and you can see the seal has split, which equals to a trans replacement anyway.
If it idles fine, and hard accell is fine, it has a high probablility of ruling out the engine or even a clogged catylitic converter as the source of the shudder. Have you though about using a used transmission? I have a friend at work that just replaced his failed 740i engine that he bought a replacement online for a great deal. Which included a warrenty. I could find out the website for you if you would like to look into it.
My gut tells me...tranny.
I hope this information is helpful. Please let me know if I can clarify anything for you. Please accept this answer. Please leave a short positive comment on feedback. Thank you.
BMW Mechanic
Level 1 BMW Master Certification, 9 Years Dealer experience, Team Leader, Shop Foreman