Everyone is telling you the same thing, you need to see the construction behind the existing arch to understand how to go forward. It's going to be filled at the least, skimmed at best.
There's no point putting 'double sided screws' for the top struts if you are drilling into a scrap of plasterboard that is making up the curve. There is no strength. It might hold. Might not.
You
could do it properly
I'd take the bottom of the arch off. Stop faffing about and trying to do half a job. It's plasterboard and pine timber framing not fine furniture making.
Now you know what you are dealing with.
Square it off with timber and battens but now you are punching screws in to other timber so it's strong and durable. Not plasterboard and void. Now you don't need
magical double ended gravity screws, and you sleep well at night. :wink:
When it's square, board out the little arch and fill. Toupret is the best fine filler I have ever used. Sorry polyfilla ya loser
. But for this I'd buy a Gyproc bagged product. I'd also have a think about buying a fine plastering bead for the top. Tack the thinnest one on you can find and if you have a square trowel you should make a decent job of it by the second try to allow for shrinkage of the first coat. Have some sandpaper to hand. And more filler.
There is less work doing it like this than trying to bodge it. My honest opinion at least in the state it is in now, needing decoration and stuff anyway.
If it was my house I know what I would do.
I'd do it properly.
Stop thinking how you can get away with it.
No disrespect meant.
Cheers
Chris