I have a crowbar slung over the shoulder on a strap. When combat happens, and I cycle through my weapons, it never comes up. So I guess a weapon cannot be used this way? Must be in the hand for combat?
Is this the same for the rifle, too?

You can access inventory while in combat. It depents on the combat itself: While you are in melee range of the opponent, or while you are beeing shot at without taking cover, you won't be able to access it. If beeing in a save position, you should be able to access inventory and then you should be able to take a shouldered weapon into your hands and to use it.

Yes, weapons are only considered equipped when held in the hand. However, as Seraphin84 says, you can access your inventory or camp any time you want, as long as you are not within your enemy's attack range (whatever that is, depending on your enemy's weapons). To access inventory while withing attack range you need to take cover. Just remember that you can't attack on the same round that you change weapons. You can use the rest of your options (dodge, move away, etc) but you can't attack for that one round.

Ohhh, I didn't know that. I tried to get into inventory once during combat and couldn't, so I thought it wasn't possible at all.
Well, this will makes things a lot easier, haha!