A revelation in the mystery of the rusting bottom plate.
After cooking some chicken tonight, I thought I'd wipe the AF out while it was still hot inside. I removed all the trays, including the bottom crumb tray, and there was a puddle of water between those same two rivets.
Apparently the steam created from the chicken, drains down the side of the AF, and into the very bottom. To it's lowest point. And just sits there, blocked by the crumb tray. And no way to get any wind & heat mixed. And since the top of the crumb tray rarely gets dirty, I don't normally remove it when I'm cleaning the grill tray or the drip tray.
After wiping up the water, I noticed little white corrosion spots already forming.