1)Always use elastic
2)It's fine to glue them. Use E-6000 and you can easily take it apart and redo it if you need to. It will still hold up to trooping.
3)No strap between shoulder bell and bicep on real armour
3b) Biceps stay up by floating on top of the forearms. Forearms stay on by themselves.
4)The purpose of the strap that goes across the inside bottom of the shoulder bell is to keep it close to your arm and not flop around as you move.
5)Not very tight, but no very loose either. It's no big deal and don't worry to much about it. You will notice if it's too much in either direction. And use glue when you put it in.