You could probably remove the window and even take out the base to make it into a doorway without affecting the structural integrity (obviously check with a structural engineer). The most common, and efficient, way of getting light into that dining space is by putting rooflights (e.g. velux) on the lean-to extension. This would allow light to come down from the ceiling in through where the window is. It also means you don't need windows on the side wall where they could overlook your neighbour. Rooflights give you plenty of light and maintain privacy for everyone. There are lots of examples on Houzz once you know what you're looking for. All the best with it.

😂 I didn’t think envelope window sounded quite right! i can only see just over half of the sofa, is the other end against the wall? Is the sofa staying there? I only ask as personally would want the sofa to align with the window I also like the idea of panelling but MCM style, there are a few MCM windows on here, , hopefully the link works https://www.houzz.com/photos/wood-panel-wall-midcentury-phbr0lbl-bl~l_170215

In fact we did, and we are very happy with it. I designed the solution and we found a good carpenter who built it for us. It's now bifold windows over the countertop and a bifold door next to it, allowing us to better separate the kitchen from living room, while able to completely open it up for an open plan feeling. I'm happy to provide more details if of interest

They do their own fixing (one link below), or check the broad category of product and find something similar. https://www.tilingsuppliesdirect.co.uk/product/jackoboard-board-fix-adhesive-290ml#.ZANJzi-nxvI Looking at your “stripped out” image above, that looks like a serious botch job. I would consider finding a cnc wood cutting company and get them to mill some perfectly circular forms in ply, say, to place into the hole and attach your flexible board to. You could get these made in 3 or 4 sections to screw together. Couldn’t find quite the right image, but this might give some food for thought. https://pin.it/6nzJppe