On the collar, I would certainly do a two-piece top-collar, as this requires less stretching than a one-piece. There is at least one tutorial on YouTube for this.
Keep in mind, that linen is not as malleable as wool cloth. So, shrinking and stretching will be a lot more difficult or even not possible.Apart from that: Adjust your lining and canvasses to the weight and the weave. Rather pointless, using a tightly woven lining or canvas, if you want your suit to be breathing.
I guess you mean at the basis of the trousers, or rather: at the fork?Yes, I'd reduce that as far as possible. If you keep it, you'd possibly have to cut it every few centimeters, otherwise you wouldn't be able to have it lying flat.
You made some trousers for yourself already, so you should have a working pattern, so you can discard the inlays mostly.
On the collar, I would certainly do a two-piece top-collar, as this requires less stretching than a one-piece. There is at least one tutorial on YouTube for this.
Keep in mind, that linen is not as malleable as wool cloth. So, shrinking and stretching will be a lot more difficult or even not possible. Apart from that: Adjust your lining and canvasses to the weight and the weave. Rather pointless, using a tightly woven lining or canvas, if you want your suit to be breathing.
Have fun!