I'm going to go out on a limb and guess that Standard-IA is basically Standard except with slow disks instead of fast. Slow disks have lower $/GB, but higher $/IOPS.
Seems unlikely, given some of the other properties it has; more likely, Standard-IA has fewer copies of the data, and then a backup in Glacier, which explains why it has high durability but low availability.
Doubtful. Almost certainly the same storage backend, but IA requests are more aggressively throttled, so there's more chance of requests being rejected. S3's capacity planning then requires less request processing infrastructure to serve IA, so it's cheaper to provide than regular S3.