You really have to think about it in a certain way. First of all, think of it as two coats patched together along the chest. The top half is a DB, shawl collared coat (grey). The bottom half is a faux-db panel coat (purple.
For the top half, it's fairly straightforward; DB coat, shawl collar with a high roll line. The top section should end around 2/3 of the way down the arm hole, which makes for a convenient front-pitch for the sleeve. The roll line should stop a little bit above the bottom, maybe 1/2"? Some sort of snap or hook/eye closure would be useful for it. There are plenty of patterns you could use as a base, I've seen other seagulls saying they started from Vogue's 8346.
The lower half is a bit fiddly. Four panels, which are only attached up a bit below the waist- below that they hang free of each other. The front panel would match up to the front side of the top, only there won't be any DB overlapping for the panels. You'll want either a hidden zipper on the (wearer's) front right, or hooks/eyes. For those little X stitches, the best way to do it would be to put eyelets on the panels and thread some thick thread or leather strips through them, so they can be taken off/put on when you open/close the coat front.
Lining would be done like a normal coat- make she shell, make the lining, attach them together at the top and front (so the hidden zipper just pokes out), then turn up the hems of each and handstitch together at the bottoms.
It's a weird thing to convey, and I've done my best, but I'm sure something's not come across so just let me know what I've mangled.