Why not, instead of thinking about the first as decelerator blocks, you consider them as directional accelerators but pointed backwards? This would simplify the utility for those clamoring for an accelerator block.
They add some constant acceleration factor in the direction they're pointing. The first example shown would then be a minecart reversing direction.
As for the break blocks, I like the idea of picking a pre- or post- stop. It gives some more flexibility for starting and stopping mechanisms that an empty block doesn't. I like it.
Also, and I haven't given this too much thought yet, but directional stops and reverses, combined with the detector blocks and some SR-latches, the crafty redstone enthusiast could probably build the first minecart turing machine.