Let me say this ahead of time. This explanation will probably come across as clear as mud.
I sew my straps into a seam where the pack body and pack extension go together. I'll space them anywhere from 2.5" to 3" apart from each other. That is 1 1/4" to 1.5" each way from the centerline of the pack.
My pack straps are angled outward somewhat. From their point of attachment there is approximately one half inch offset from the side of the strap closet to the center line to the side of the strap closest to the side of the pack. This results in the ladderlock end of the strap ending up just a little outside of the pack body.
My straps are curved but not S shaped. They are relatively straight for the first 2/3rds and then curve outwards at the ends.
In theory the straps should only curve over your shoulders and downward in front, around your torso as they make their way back to the webbing that is used to adjust the straps. From the pack to your shoulders there should be an air gap of sorts. There should be some space where you could see daylight. Ideally the straps should leave the pack at something close to 90 degrees and not start curving until they go over your shoulders and downwards towards the adjustment webbing.
My most comfortable pack to date had 3" wide shoulder straps with 3/8" CCF padding. My latest pack has 2.5" straps and 3/8" CCF padding but there is also a padded removable hip belt on this pack.
"...or do the straps form to your shoulder's slope regardless of the angle of attachment"?
Barring any outrageous angles or 90 degree perfectly straight attachment my answer is yes. The straps should more or less form themselves to your shoulders.
"The XPAC fabric that I used didn't like being turned up into an arcing seam..."
All of my pack's shoulder straps have been sewn in using straight seams even though they were angled outwards.
Take a look at Jay Ham's article on 5 Yards to SUL, tarp, pack and stuff sack in the MYOG articles section.
I hope this helps.