Attached is a very basic diagram of interfacing a coat. The sections A are the lighter weight interfacing and C the heavier weight to strengthen and shape the shoulder to reinforce the shoulder. Nearly all women have a hollow just under the front shoulder and this heavier weight allows the fabric to skim over this. Ignore B and the tape on the roll line this shows a revere, just take A upto the CF.
As for the collar, although soft it rolls. The roll line will either be marked on the pattern or you will have to work it out, or is it on a collar stand? If it’s a collar stand fuse it with the heavier weight but if not you’ll have to mark one. Mark up the CB of the collar about 1 to 1.5 inches and curve round to the front edge. If you fold the collar over your hands and then curve round as if it’s round your neck this should give you an idea if it’s in the right place. The bottom part of the collar up to the roll line is in heavier weight and the top part is in lighter weight. Only the under collar is interfaced usually but if you want the collar a bit stiffer interface the upper collar in the lighter weight. The lighter is cut on bias and the heavier on the grain.
From experience I wouldn’t use fusible as a sew in as I’ve found that it makes the seams stiff because you can’t pair down as you would a sew in and also when you come to press, the fusible attaches to the fabric and bubbles can form. All fusible interfacing shouldn’t go into the seam allowance.
Another tip from experience interface all pieces before sewing the seams. I didn’t when I made my coat and couldn’t get the interfacing to lie flat. The front was in three pieces, so I thought sew it together then interface the whole lot, didn’t work. The same goes for darts, interface around the dart markings and then sew.
Hope that helps.