Fred - Great discussion.
I have to wonder though - if only 5% of the acceleration occured over 95% of the stroke, I would have to question whether there would be adequate tension in the line by the time it reached the power snap/tuck/SUAS especially if the caster is aerilializing a lot of line. Lack of tension (from acceleration) would let a long aerialized line fall (gravity), causing problems.
In two-handed overhead casting, there is no double haul so tension in the straight line coming forward is critical. I accelerate more throughout the cast to get the distance I need fishing these rods in tough conditions like surf, very tight loop by stopping abruptly.
In spey casting, lack of tension in the beginning of the forward cast can result in the d-loop falling downward or the anchor becoming excessive and killing the cast. This is particularly true with long belly spey lines, so there is a correlation with line length and early acceleration. Extended belly line technique relies upon early acceleration in order to make the casting effort reasonable.
However I get your point, there is lead before speed and it would be interesting to know what ratios actually are, and the effect of more acceleration prior to the snap, etc. I sure don't know what these are in any specific terms but it would be a great physics exercise.
What I do know is that extending the stroke when done right puts a deeper bend into the rod and increases the length of line that is under tension coming forward. This makes the casting motion feel stable and smoother, and casting is definitely all about feel. The line has more momentum with a longer stroke and good pre-loading acceleration.
As far as straight line path of the rod tip, I stay focused on the line being pulled into a very straight vector under tension following the rod tip, which BTW is moving straight. It's easy to see and adjust the line's path but it's hard to watch the tip of the rod. I assume when the line travels in a very straight line and the top half of the loop is very straight then the rod tip must be moving correctly.
You're right it is kind of odd to think of the rod tip as traveling in a straight line but if you think about it, it must be in order to lead the line straight. The directed length of straight line under tension becomes the top half of the loop, and we know this has to be straight for a good cast. The rod tip needs to travel in a straight line to create a straight cast. The rod tip deflects in a way throughout the casting stroke that the tip draws a line straight enough to pull the line under tension in a straight path. The tuck of the thumb and lowering of the hand as it comes forward helps keep this all in line.
Of course there is a semi-circular "tuck" of the tip at the end of the stroke, which has a lot to do with the loop size. But even when the rod is bent hard during the majority of the cast the rod tip should keep the line moving straight, and to do that it has to be moving in a straight path (see below).
I really like Cathy Beck's metaphor of taping a pencil on the tip of the rod drawing a straight line on a white wall, which is mostly used to talk about the casting plane but can be applied to the straight path as well.