Concerning your color question of course a primer coat is recommended for the best finish. But if you wanted to not use a primer coat you'd more than likely need additional coats of color to cover up the prior paint. It won't distort the color, it will just take more coats to cover up. As for using a sander you can, but it won't give you as smooth of a finish because it will jump around the car slightly giving it an uneven sanding job. It will definitely speed things up though and the difference may not be that noticeable.
Other colors work just fine but darker colors will show imperfections more. And yes, humidity will cause it to take longer to dry and may affect the application some. Just be mindful of the fact.
I'm sure you could easily add something for more shine and protection, but I can't offer any advice on doing so.
If you're happy with how it looks now I wouldn't mess with it. But I would put on a clear coat to help keep it looking good longer.
I found that the roller applied too much paint at once, leading to the need to sand heavily between coats. With the brush you get a lot more control of how much paint is applied and where.
Colorado warm... 70* or more. Part of the key of this paint process is to not paint in direct sunlight as it will cause the paint to dry too quickly. If you're in a smokin' state such as Arizona you'll definitely want to do your painting in a well shaded area.
I have no experience with any other paints so sadly I cannot recommend another paint or brand for you. I'm sure a local paint store can assist you with this question.
Excellent write up! I wish I would have thought about this as it seems my old (I just moved) swamp cooler was the damn same as yours. I was always grabbing the hose and spraying that pads to get a boost of cool.
Invisible Book Shelf
Car Dash Camera Mount
The Poor Mans Paint Job