Not sure about your question of "press open the seems"?
But do know that wool in general is prone to fraying so the Blanket stitch is common. Also the Bar Tacking is highly recommended, I had to add a few on the one Northwest Traders made me...but not until after a couple years of use when i noticed stress at each end of the hood attachment.
I would ne interested to hear what you find out about your Seems question.
Would also like to figure out how they do the 'Hidden Stitch' for attaching the inside pocket (it's so 'hidden' I can't see it to figure it out).
The question was if anyone has done it. Press (as in iron) open the seams, (not seems).
If you do not understand it, I'll take that as a no. And maybe it isn't common with capotes and wool blankets, but I've run into "press open seams" a lot in my research. It's used as a way to flatten things and seat the stitching. I just haven't found where they do it to heavier wool blankets or not. Like most stitching tutorials, I've only seen examples on test material and not actual product material.
If the hidden stitch is the same as the invisible stitch, or ladder stitch that was mentioned here several times, It seems like a lot more work to me. If I understand it right, you need to hem the edges first so you can use the ladder stitch. Nobody really elaborated on it other than mentioning it and posting video on sample cloth and not actually using it on attaching a sleeve to a shoulder.
If you think about it, if you hem 1/2" on both the top of the sleeve and the body where the sleeve gets sewn so you can use a ladder stitch, you have just changed the circumference where they sew together by 1". Let alone shrinking the sleeve when you hemmed both edges down the length. How do you fudge that together?
Disclaimer: just trying to get all my ducks in a row. I'm a measure twice cut once kind of guy.
Not having any experience in this it seems like hemming things before sewing them up could alter measurements.