No option to build a main hall in Skyrim

None of the characters make a "better" steward; they all behave exactly the same in that role. They also still work as followers: you can still ask them to follow you again, marry them, etc.

The only thing that changes when you make a follower a steward is where they hang out when they stop following you. Like the housecarl for the appropriate hold, your steward will wander around the inside and outside of your house when idle. This means they'll be hanging around when your house inevitably gets attacked by bandits, giants, or whatever else. This brings up a couple of mostly minor considerations:

• Your steward is often the first line of defense from attacks; they'll jump into combat as soon as you get within range of the house, often before you've even noticed the enemies.
• If you pick someone other than the assigned housecarl to be steward, you'll have two people hanging around guarding your house instead of just one.
• If you pick a housecarl from another hold as your steward, they leave the original house empty. This has no effect at all on gameplay but it bothered me in a slightly OCD way.

I have found a good pattern is to pick up a mercenary, get them to the level cap, then leave them as steward. I also take the housecarl out until they hit their cap and let them go again, giving you a lvl 40 and lvl 50 character hanging around your house to help fend off bandits.

Sounds like a bug. I would reload a save from before buying the second plot of land and start over.