Florencia and Jana (by email) both asked about the lapped, flat-felled split side seam, thank you for asking! Since I finished all my Christmas sewing today... and, can I just say; Whoopeeeeeeeeeee!!!! I had time to get out some scraps and show how I did them...
Bear in mind that a tiny bit of the wrong side of the fabric will show in this technique, so it is best confined to fabrics where this doesn't matter.
To have this feature, you have to plan ahead and cut an extra little extension, or tab, on the outer edge of either front or back; about 11.5cm in height for a 10cm split. I randomly chose for the front edge to have the tab so as to flat-fell the seams towards the back as my own convention; but it looks equally fine either way.
Stitch the side seams to level with the top edge of the tab.
Press the side seam open above the split, at the same time press down the seam allowance on the un-tabbed side and leave the tab un-pressed. Snip the untabbed seam allowance to the endpoint of the previous stitching.
Fold under and press the untabbed seam allowance below the snip, and stitch.
Trim the remainder of the untabbed seam allowance to half its width, along the entire length of the seam.
Press the tabbed seam allowance over the trimmed seam allowance, and wrap the raw edge around the trimmed edge of the other side to enclose it, the entire length of the seam. At the same time, press the tab under along the same line to the lower edge. Pin in place.
Stitch the flat-felled seam, stopping at the top edge of the split.
If necessary, trim the tab; then turn under the raw edge and press. Stitch.
To strengthen, stitch a short row of horizontal back and forth over the top of the split. Alternatively bar-tacking, or narrow, closely spaced zig-zagging can be used at this point.
Hem the lower edges. Remember: the underside split is hemmed facing out, so mind you don't accidentally stitch the hem on the outside on this side! I almost got caught here the first time!
Finished views: below left, the inside; and at right, the outside view