@Ploshkin
With the interfacing for the buttons and buttonholes, will it matter if it's attached to the main front piece or the front facing (identical pieces cut from the main fabric)? I've been puzzling it out in my head and I can't work it out! Just because I've already attached the interlining to the main pieces so I'd rather not remove it if I can just interface the facing.
The usual way is to cut away space around openings, pockets , darts, pleats and anywhere that there will be bulk or detail.
For bound buttonholes, when the facing is the same fabric as the shell, then interface both.
The purpose of the interfacing in this case is to stabilise the fabric and to support the stitches for the bound buttonholes. I like to interface with a light or medium, knit fusible and then use a letterbox method with silk organza to create the openings.
The interfacing you use and how depends on your preferred method for making bound buttonholes.
Interlining is an extra layer for warmth and often swings free between the interfaced shell and the lining.
It sounds as though you have underlined the shell fabric with the warmth fabric. It depends on what you've used for the interlining as to whether it will also do the job of interfacing which is to support the shell fabric.
Either way, as mentioned above, you cut away sections where you have to reduce bulk.