First time poster (and what better way to start than by asking the author of my favorite books some questions?)
1. When Scadrial was closer to the sun, can we safely assume that the middle section of the planet was scorched clean of anything living? Could there have been some underground life thing going on? Anything cool or interesting sitting out there (like ruins or some lost technology)?
The middle section was scorched pretty clean. I know of a few interesting tidbits, but it's not technology. (The tech level before the Lord Ruler took over was nothing particularly special, early industrial era.) The cool and interesting things are on the southern continent.
2. What's up at the south end of the world (during the 'closer to the sun' phase)? Life there? Cultures? Allomancers? Assuming that there is some life down there, can we assume that we'll have some interesting 'culture clashes' in future books?
They will be known by the modern trilogy, so it's safe to assume that a discovery will happen soon. Either during the Alloy of Law era or soon after.
3. What's the closest that humans had gotten to the 'inhabitable' zone of the planet during the events of the first Mistborn trilogy?
There were groups who would go out there to escape the Lord Ruler, and the Final Empire in general. Survival was practically impossible. It's possible someone might have gotten across to the southern continent, but it would take a small miracle.
1. Dalinar ever getting an honorspren?
RAFO. (You knew I'd do that.)
2. Is there a functional/structural difference between modern-day Shardplate and the stuff the Radiants wore? Did the Radiants have to use infused gems to keep their suits going or could they just 'breathe in' Stormlight and feed the suit off of their 'inhaled' reserves?
Something is different. You will find out what.
Uhh...guess that's it. I'm sure I'll kick myself later for forgetting one, but I guess I'd never be done with this post at that rate.
Thanks for reading!