Seats means persons, how many persons may use the add-on, if you are an individual user, you can install it on your Linux, Windows, and macOS, since there is only one person may use the add-on.

They are different licenses for different teams. For example, if a small team which has up to 20 artists who need to install the add-on, the team should purchase one license for 20 seats, it is much cheaper than purchasing 20 indie licenses, and the team can't purchase only one indie license and distribute the add-on to all their artists, for it is illegal in law.

It depends on your armature and 3d models.

For simple armature and 3d models, the 'High' quality works well, you can even try the 'Medium' or the 'Low' quality.

But for complicated armature and 3d models, you can try with the 'Highest' quality, if it is too slow, try the 'Very High' quality, then the 'High' quality, until you can balance the quality and the speed.

If you use Blender's default system unit - In meters, the default 'Deviation Threshold' equals one millimeter, which is suitable for most cases.

But if the 3d model is too large or too small, you need to increase or decrease the value by experiment.

If you do not use Blender's default system unit, such as in centimeters or in inches, please also use the default value, it seems that Blender always calculates in meters.

If you can't find a proper value, you can simply bake vertex weights for all vertices.

Yes, you can have a try.

Bigger twist angle will distort the 3d model more heavily, which may make the result better, or make the result worse.

You can get a proper value by experiment.

Yes, you can.

Please setup the bake range to 'Selected Vertices'.

Select the mesh object, enter 'EDIT' mode, select those vertices you want to bake, exit 'EDIT' mode.

Select both the mesh object and the armature, start to bake, it only bakes vertex weights for selected vertices, other vertices will not be touched.