I'll offer what I might do...
Are there pockets or is that just a seam?
I guess it really doesn't matter cause you can't preserve them and take in the side seam too. You can always add pockets after you get the coat resized.
Is there a lining?
You could move those buttons from the side to the front for the double breasted effect. And remember you may not be taking it in as much as you may think if you are going to stick with the double breasted style. Or are you just showing that pic for the A line part?
I think I would have left the sleeves... that's because it would be easier for me to alter that seam from the sleeves through the underarm and down to the body. It's easier to do that than to replace sleeves and have them fit right cause you are going to be taking in the side seam and the sleeve separately then putting them back together. If you take them both in in one seam then they HAVE to fit each other. See?
You should have no trouble getting the A line shape. It's one of the easiest shapes to acomplish. Lay it out flat, eyeball it in relation to the picture you have there and your body and mark it. Mark it by laying it on the floor so you can look down on it. Use something like skinny strips of black fabric or cord or something. Lay it down on the coat then look at it from above.
Turn it inside out and put it on. Take pins and start pinning it and do that till you like the way it feels when you walk around. Remember that the underarm seam will be bunchy when you pin it (be careful with pins in the underarm) so until it gets sewn and then trimmed you will have to judge how much to take in and trim off... You could even trim some of it, pin it and then do that gradually till you can tell how much to take in...
Remember to be careful not to trim too much. You can always take more off. It's real hard to put some back
When you do get the seams where you want them use a good overcast stitch or a straight stitch then a zig zag stitch right next to, or just barely catching that. That will make for a nice flat and strong seam.
Remember... Only trim the access of the seam all the way off when you have finally gotten the fit that you want. Press the seam flat and you will be very happy with your new garment..
That's a kick ass coat. I know you will make it look good.
I hope this helps some