I'm not sure of this lacking any real scale to determine screw size but those screws in the window frame might have a thicker shaft than the stock rivet. Could have been an adaption to drilling out old rivet and having a hole too large for the stock rivet to grip.
Just a thought that might explain the screws there while other places are still rivets.
Oh and white liquid electrical
tape can be dabbed on a rivet as a short term seal. Lasts 6 months or so around here and seals things until you get around to replacing them. Big box stores will have liquid electrical
tape, white matches pretty well.