You don't want the Force-a-Nature or Backburner. They are dropped randomly in answer to your question.
Force-a-Nature is for failscouts. It's like the giant easy button for the class. If you want to be unskilled as scout and lazy, take it, but don't expect to help your team.
Backburner is for failpyros. Not that they weren't already the way they are played. Regular flamethrower is best, as you can put out teammates or blow away projectiles. The only upside is that if you like ambushing, you can get crits, but that's it. No other benefits.
Can't say about the screens.
And for the love of god, do not use the Huntsman. Sniper - Sniper rifle. There's a reason for that. Huntsman, or rather, the Cuntsman, forces snipers to do what they shouldn't. Attack. Although some handicapped snipers still attack. Don't.
