[Christ]

You can't start the car in gear for 2 reasons:

1 - It's dangerous. The engine could rev unexpectedly after/during starting, and cause the vehicle to move.

2 - The engine is already a huge load for the starter. Add in the transmission and fluid pump... pumping fluid, and you've got even more load. All the while, it's trying to move the car, perpetrating #1.

You could bypass the Neutral Start Switch, but I don't recommend it.

BTW - Park is just Neutral with a locking pin.

There is nothing wrong with downshifting in an automatic. In fact, it's recommended to save engine/transmission wear and brake linings. If you're going down a grade, or slowing down, downshifting works exactly the same as it does in a manual. You don't always need to use your brakes to slow, often, you can move to the next lowest gear, ready your brakes, and use engine compression to slow the vehicle to an appropriate speed for the situation you're negotiating.

If you find that your transmission is shifting up and down frequently, such as in hilly terrain, it's best to shift manually into the next lowest gear, or slow down to prevent it. Each shift represents clutch wear and fluid shear, which creates heat. Often, worn parts aren't the cause of a transmission failure, instead, it's bad/burnt/overworked fluid which allows other parts to self-destruct.