The spec said the width is 39.5" and depth is 4.75". But the sketch one of the photos shows the width as 29.5" and depth is 5.0". Pls confirm the exact dimension.
For the Medium medicine cabinet H 26"x W 29.5" D x5"

2) What is the sectional dimension of each mirror piece in the front?
The Larger section is 19.5 and the smaller is 10"
3) Can the med cab be installed invertedly, i.e. the shorter width section be on the left side, and the longer width section be on the right?
Yes it is possible to reverse the doors or turn the cabinet upside down to flip direction.

4) Is there any opportunity to mount a small magnifying mirror inside the med cab? Any photo of the interior med cab when open in full?
There is really no space for that, one of the shelves inside would need to be removed or adjusted and you would loose a lot of space, but depends what you are looking for, it may work for you.

Thank you for asking i hope this helps.