Oh your hurting head!
Sometimes our overthinking is for the good though, as we are trying to prevent disappointment.
Another factor to remember in your sizing ... shrinkage.
Be sure to allow enough excess in your quilt, so you will still have the overhang you want.
Separate to that, I would want my quilt more than the 2" extra you mentioned. That would mean I would have to have the quilt sitting "just so", to be sure the coverage was there. I don't like fiddling, so would allow for more than the 2". Though some quilts have a design/borders that you soon know what has to sit along the edge of the bed for it to all line up. Remember, that's me ... not necessarily you!
To check what size you want it, and see how the overhang would be on the corners,
I would take a quilt and pull it over one corner to the drape length I wanted.
Measure and then calculate the finished size you need.
Bed Skirts can be very simple to quite deluxe.
Simple gathers or box pleats all round.
Or even just one box pleat on each side and end and at the corners.
I have seen them done with fabric just draped down, and no extra fullness in anyway.
I think if I were doing that, I would use a heavier fabric.
As for me ... nope, can't be bothered with bedskirts anymore.
I just have my quilt/bedspread hang over as low as I can.
Good Luck!