[Underbelly doors]

What is the purpose of these doors? are these supposed to be opend during fligth?

Please refer to the doors right at the letter "E"

[Inibuilds A300 MSFS Trim over sensitive]

Yup... Same here

[Inibuilds A300 MSFS Plane still dropping out of the skies before touch down]

I concur. It also happens to me. I've also  noticed that after touch down, it tends to lean to either the left or the right mains, causing me to adjust with aileron input.

It happens no matter the airport or weather conditions.

[Excessive landing gear cycling time]

Yes it does... I would say the Flaps cycle as well...