Keep in mind the tub can't hang from the ledger...it must be supported well from the bottom, the ledgers are to prevent the edges moving if you do something like sit on it and to help keep them straight.
Ideally, you'd notch the studs so the tiling flange would be flush with the studs. Then, you can install the cbu over the tiling flange to about 1/4" from the tub surface. you want a vapor barrier behind the cbu, lapped over the tiling flange.
If you don't remove the drywall or cbu first, it will be very difficult getting the tub into the hole. With the skirt, you'll either need an access hole in the wall or access from below to make the final connections. Depending on the tub and the drain, it may end up longer with the drain installed, so you couldn't slide it in, plus, since you can't drop it in if you don't have the walls free, you'd not be able to have the drain/overflow assembly installed first.