Fixing widow frame corners with unis?

The corners of large fixed windows have a triangle wedge glued in the corner to make the radius. If I remove triangle, then dish out say 4” either side of the corner with a Dremel, then fill the void with glass (or carbon) unis, would this be a good fix?

 

I think you are over-complicating it. Use a medium density filler with epoxy to fill the void. In the round corner where the core appears to show, 10 to 14 oz cloth would be enough.

 

I tend to overthink things. I often lie sleepless in bed thinking of a better way to tie my shoes.
I ended up wetting out a bunch of unis and stuffing the void until just below flush. It might be overkill, but I’m installing new windows and painting, so I don’t want the cracks to come back again.

 

If your reinforcement fillers had a high percentage of milled fibers, you'll be fine.

 

What’s your take on the cracks in this opening? When sailing upwind, I could see the cracks open and close slightly.

 

It's probably the deck cap and/or liner flexing underway, suggesting the laminate was made too thin or you have a structural issue somewhere. This isn't all that uncommon on large fixed ports or lights. Check the tabbing on roof and deck beams in the general area. Any cracks or tabbing that looks likes it's moved? I saw a large sailboat with is on all of it's oversize, stylized, quick radius corner lights (fixed window). The roof had foam beams, tabbed with a few layers of roving, some of which had delaminated. The three "beams" where replaced with Cossa and tabbed back in with biax, solving the issue.

Your current repair will hold for a while, but if there's much movement, it'll start to crack again, probably along the edges of the areas you've just repaired, at first.

 

This is an old repair from a previous owner. He dremelled the crack and filled with thickened epoxy. Needless to say, it failed again.
I’d like to make permanent repair. I’m thinking to stiffen opening with unis in the corners (like the other windows). Then add lots of glass over the cracked area.

 

Those are clearly stress riser cracks and in the obvious locations for them. This is a common issue for engineers to work around. Again, though previous repairs have been made, they haven't solved the issue(s), so much as place a band-aid on the symptoms. You may be correct in that you can grind down the areas affected, place a few layers of biax and fair it back up, though if the local flex in the area is enough, it'll just come back or may manifest itself with more cracks in other areas, not yet effected.