[sharkbit]

Try this-one of the best tutorials on manual targeting:
http://www.paulwasserman.net/SHIII/

So - if I then say, follow a course change from the target, I would have to re-enter the details on the TDC (F6) screen despite the fact range, AOB and speed would all still be correct?

If I understand your question, the target changing course will cause the actual AOB to change and will need to be updated once he settles in on his new course. Speed should remain constant, and of course range is always changing and needs to be set accurately at the time of shooting. (Unless you are shooting with a 0 gyro angle and an AOB of 90-it will make more sense after reading the above tutorial-I don't want to confuse the issue yet ).
A course change by your sub will not change the target's actual AOB but will make the AOB setting in the TDC inaccurate and will need to be reset.
The reason is if you move the scope/UZO, the bearing to target changes which changes the AOB setting automatically in the TDC.

To see this, with the TDC turned on, move the scope/UZO and the setting in the TDC will change.

And - If I were floating around still on my way to my planned attack point and was not facing the correct direction yet, and at this point entered AOB of 90 degrees, when I arrived and faced the correct direction, any data entered would be useless?

Yes. Any course change by you or the target makes your AOB setting in the TDC useless.

Finally - The bearing setting (which I understand does its thing automatically now) is attached to the sub compass, and this is the reason that AOB cannot simply be set to 90 degrees at any time, because this would ultimately skew the results if not already in the desired position?

You are correct. I think it will definitely make more sense after you read the tutorial.

I hope I have not totally confused you and have answered your questions.
Feel free to ask as many questions as you want. There are a ton of people who are more than willing to answer them and may make it even clearer than I can.