I'm thinking of using wings to make backpack-like skins, such as scuba tank, gliders (static wings) etc. For example: I tried to mimic a Mekanism jetpack.

But the chest bounding box is limited.

Then, the wing addition has opened that possibility. I found out I can set wing angles to zero to disable the flapping animation, and place the markers anywhere. My question is, since I'm making something not expected, can I make wings this way without any problem?

Suggestion: An "animated wings?" checkbox in the Skin Settings tab would enable/disable the wing animation options (speed and angles) and dismiss the need of markers.

Nitpick suggestion: is it possible to align the torso model in the wing mode with the torso model in the chest mode? Because if I want to use the chest skin as base for wing skin, the skin will be 2 blocks off N-S ward.

Yes setting both angles to the same value will work fine. If you are having issue setting them to the same value, there is a preview build on the discord server that has fine tune buttons and a bigger wing size. https://discord.gg/SMjJj5R

and yes I will fix the offset, I will also fix the sword/bow offset.