Great write-up and isolation points you've offered up.
Headlights bright and no interior lights are great to know.
Not very likely the key pellet or key at all.
OK sure sounds like corrosion on the main acc bus post
but could also be corrosion at the battery ground.
The acc bus post is located just next to the battery. It's just
to the right of the battery (toward the firewall).
Here is a pic of the area (battery and battery base removed) but the post is just off screen to the right.
You may need to remove all the lugs off the acc bus post, clean each then re-attach all lugs applying
dielectric grease to all.
First, do
measure the new battery voltage right at the posts as someone turns the key. If it drops below 11V your new battery ain't worth squat.