Why not just cut your material an inch or less larger all the way around, iron the edges of your material under, cut a piece of batting slightly smaller then lay them on the seat and tack directly through the folded edges of the fabric. A little spray glue under the batting or between layers if you use 2, wouldn't hurt. To make a tidy corner: fold the excess fabric/seam allowance accross the actual point* then fold down the sides. *I looked for a pic and couldn't find one. another way of saying it: take the corner of the fabric piece and fold it down so the point is pointing inward, and the fold is at the point where you want your finished corner to be. Then fold in the sides. Good luck and post a pic. I love windowseats. Kathy

There are different "types" of microfiber; I would test it before you buy it. I had a settee/bench from Crate & Barrel that cleaned up easily with baby wipes (it was next to my kids "art center" and was stained with paint, markers, ballpoint pen, etc...). I know someone else who had a microfiber couch that spotted when you cleaned it. Have you considered leather? We have leather Pullman barstools and chairs from Crate & Barrel for our dining table. They are 4+ years old and have taken much abuse (similar to your situation) and have no stains or marks on them.

This is gorgeous, 2.625 yd Brunschwig Fils Upholstery Fabric Monsoon Floral Coral and I think it would work beautifully, but note the odd sizes of the pieces if you are interested. Brunschwig fabric is very expensive, and this is a 'relative bargain' on eBay.

Hope you get an answer. Chairs are great with the Victorian lyre back style. With the seat notched, one would assume it was to drop it into the frame, and then secure it. and once the screws were out you could apply pressure to the under carriage and push it straight up. The frame of the chair is one piece? No other pins showing? Have you tried another chair? Could one be just stubborn? I did find a couple of posts about seats which cannot be removed. It may address your problem as your seats look like there is a welting around the edges. Hope it solves your dilemma. https://www.dimplesandtangles.com/2019/08/how-to-re-cover-upholstered-non.html