Getting my windshield replaced, and though most of the rust underneath the old gasket is shockingly minor, there's about a 6mm hole right at the top left corner that's been allowing water into the A pillar. That explains one of the nasty leaks every time it rains. The guy also says the old gasket was installed without any sealant either in the glass or in the sheet metal channels of the gasket, which explains the water coming in right at the top and dripping from the mirror when it rains. All the corrosion he'll wire wheel down, then apply a rust converter and then a urethane adhesive over the top that'll penetrate into the pores and provide a better sealing surface. Honestly I wish I could just Tig up the one hole right now and be done with it, but he says with the converter and adhesive (and sealant in the gasket) he uses it'll easily hold until it's actually convenient for me to tear into it and Tig everything. He's got a lot of experience with 60 series windshields (and definitely knows what he's talking about), so hopefully this fixes the water ingress I've been having for so long.