The clamp ring on my Campster windows
did not want to have the edges at the split in the clamp ring come back together tightly. I used a bar clamp
pressing against the top and bottom of that clamp ring framing piece just beyond where the radius starts but where the frame is still straight to bring those edges together and then installed the screws on that side of the clamp ring. After the screws were in I took the bar clamp off. Problem solved
My window's height is less than the opening width of a 24" bar clamp. Bar clamps are not all that expensive to purchase and are quite handy to have in a home workshop. Or if you have a neighbor who does woodworking you might be able to borrow one for the task.
As to the screw issue you can always wax the screw threads before installing them. My clamp ring screws were zinc phosphate coated and there were no rust problems or binding problems from them even after 45 years of being in a leaky trailer that seems to have spent many years as a garden shed. The zinc phosphate coated screws have a dull grey appearance.
Stainless steel screws can develop an issue where the threads bond to the aluminum over time which makes them difficult to remove and they can also develop rust underneath the heads if moisture gets under there. One preventative choice if using stainless screws is to put an anti-seize compound on the threads and under the head before you install the screws into the clamp ring. Or you can wax the threads of the screws.
But for the long term the zinc phosphate coated screws are likely the best choice. Don't worry about matchy, matchy, interiors with everything being designer perfect. Concern yourself with the best choice for structural integrity and longevity.
You can always paint
screw heads yourself. It is easy to do. Soak the screws in a jar of acetone to remove any oils from manufacturing or fingers, let dry. Wear gloves to keep your finger's oil off the screws after cleaning and in the next step. That step is to stick them into a piece of foam, leaving the head above the surface of the foam rather than touching it or you can screw them into a piece of corrogated cardboard if you don't have any foam around. The point is to secure the screws in an upright position so they don't move and have them far enough apart that the paint
will easily coat the sides of the head as well as the top. Don't worry about coating under the head, some spray will likely get slightly underneath which is OK.
Use a couple of light
coats of automotive spray paint
. One coat of primer first. You can buy black primer so that you only need one coat of paint. Do not use heavy coats of paint or you will fill up the slots in the screw heads. Let the paint cure a couple of days before installing them. A week is even better, paint takes that long to fully harden. Be sure you use a nice new screwdriver tip so that it gets a secure grip and does not mess up your paint job.