Agree with the new screws and planned on that. It's cheap improvement. In passing you mention important point: structural screws. The temptation is stainless steel (to remove corrosion issues) but the spec is for a steel screw. Stainless is softer metal.
Going to try one of these pneumatic screw extractors (see links below, can get from aircraft spruce and others). A friend who is in maintenance at FedEx says they use it on "big iron". Has not used it on light airframes though. Second link is video shows one in use on a car. Making sure to use riveter as there is ability to vary power rather than "full on". But I am saving this for last resort trying in progressive fashion manual first, then small electric impact then this. Of course lots of "penetrating" oil. I'll crack the screw to provide a gap and then penetrating oil.
Any recommendations on penetrating oil? A little recent research showed there really isn't much difference among them. PB Blaster has been my go to usually.
Have also invested in screw head paint cutters. Included a link for that. Also from aircraft spruce.
https://www.aircraftspruce.com/catal...rewremover.php
https://www.youtube.com/watch?v=SMN_4-1nci4
https://www.aircraftspruce.com/catal...tcutteran8.php