Some units- eg the 2nd Foot definitely had lapels from the later 1720s. There is an extant life size recruiting figure from that regiment with lapels still in existence in the NAM collection I think.
As we can see from the 1742 Cloathing Book, almost all regiments had lapels by then.
Re turnbacks- contemp illustrations from the 1715-35 period eg 1st Guards, 2nd, 3rd, 11th Foot all show no turnback, as do several contemp pictures of infantry on campaign in the Jacobite Rebellion of 1745-6. However, as others have stated above, the turnbacks of those days were just that- real coat skirts which could be fastened back or left open- according to fashion, regimental taste or field conditions. 1742 is the first date I am aware of which has illustrations of turned back coats.
One uniform difference which does seem to have become more common (if not universal) between the WSS and WAS was the wearing of long gaiters over the shoes and stockings. These were usually white even in the field. The first references I am aware of to dark coloured gaiters (black, grey or brown) date from c1744.
Another difference in appearance which came in quite soon after the WSS was either cutting the soldiers' hair shorter, or sweeping it up under the hat to give the impression of a short hair style. That too makes quite a difference.
On the basis of the VERY limited evidence- I would say that MOST regiments of foot from 1715-1735 looked like this:
full skirted coat
no lapels- although lace on buttonholes and cuffs became common quite quickly
long white gaiters
short hairstyle
This adds up to an awkward mix between early and mid-c18th uniforms. Sorry…..