The first photo looks like something like a tree branch abraded away the clear coat and the aluminum oxidized there. Not sure though...maybe someone else has another idea.
The spray paint can definately be removed with paint stripper...BUT that will also most likely remove the clearcoat too. They were probably painted because they were not aluminum and had started to rust....bummer!
If you are thinking of polishing your trailer, it would probably "fix" both these minor blemihes. But it may not be worth it just to get rid of these "beauty-marks" as it's a big commitment to acheiving the polish and then maintaining it.
If not planning on polishing, I would try a little mild rubbing compound-type of wax to gently try & remove the spray paint without effecting the clearcoat (try on a hidden area first!). Then replace the rivets/screws with aluminum ones.