Tremor
The problem is my difficulty in playing the part of the tracks which I know should be "slowing down" with sufficient correctness whilst still at the higher tempo. Also if I record one track and apply the gradual change, it's very difficult when recording the next track to "sync" it to the existing one which already has the tempo changes applied.
I'm pretty sure I would have a problem with that as well.
Trying to play something different from what your brain wants to hear would be very challenging.
On your second note, that would be nigh on impossible, I think.
As you attempt to sync the second track, the first would change proportionally.
Expanding on Greg's approach, I would probably lay down a click track using SI Drums, or SD2.
Then open the Tempo map, and adjust accordingly. You can even draw in the tempo changes if you wish.
Based on your equipment list (thanks for that), I am guessing you are recording mostly MIDI tracks?
Make a few dry runs, and adjust the tempo until it fits what is in your head.
Once you have that baseline tempo mapped out, you can follow it, and play the way you want to.