I'm still thinking about the neutral/reverse/park problems. I haven't come up with a good solution, yet. Once I do, you can bet it's gonna be here, in plain type, using my normal over-expressed and extremely detailed style.
My best idea right now is to use the existing shifter to actuate between P, R, N, D.
You put the shifter in D to begin driving, but then you have to manually select gears. You could feasibly start in 4th gear if you're not careful. Once you've exited first gear, TC lock could be engaged by another micro-switch on each of the 2, 3, and 4 gates, actuated by the gear selector, or it could be manually actuated (which is less idiot-proof).
SO, to pull out of your driveway, you'd put the original selector in D; Put your manu-matic selector in 1st gate; begin accelerating; as engine speed increases to your "comfort point", remove selector from 1st gate, place selector in 2nd gate.
Wait - I see a problem here, already. If 1st gear is OFF OFF, any time you move between gears using the selector, the solenoids engage 1st gear. This would be BAD. Now, you'd need a redundancy circuit that allows the previous gear to remain active until the new gear is selected, which is beyond my capabilities.
Also - it's incredibly impractical in a manu-matic to actually pass through neutral each time you shift gears, so a "clutch" pedal, as it were, isn't really necessary, but might make a nice design feature for "comfort".