Just a point, you say sewing in pleats gets you nowhere, but I disagree at least with the Phillabeg, as it produces a better garnment (neater pleats, no scruffy overhang, it holds it shape better, higher waistline for warmth).

My hunch is that as I can see clearly the skills were there, it was a case of some using the drawcord, one person fed up of being beasted for scruffy pleats produced in a hurry daring to get his phillabeg stitched, getting away with it, others daring to do so, it being observed that it works & is more practical, someone in command realising it is actually better & making it official when faced with the choice of disciplining the whole regiment for not strictly following dress regulations or making himself seem like he's had this revolutionary idea & claiming credit for it.....

There's more than enough evidence if you look at the forces through time, especially on campaign of how dress can change based on the actions and experience of individuals eho have dared to use a piece of none issue kit or alter issue kit. I don't think a sewn phillabeg would be any different.