As someone whos played in every position in the forwards, I'd say the damage you take is different, I'll try and answer your questions.
1. In terms of the really big tackles, I'd say flankers often take the hardest hits, as opposition often build momentum, and the loose trio will get more time with the ball. However I'd say props get the most damage all together, as their easy to line up, and their job often means driving the ball forward. In terms of scrummaging, the damage isn't all ways painful, it's just you will either go backwards or forwards, and if your getting dominated you feel like your letting the team down. However scrums also have a very high risk as it's been pointed out, especially when a scrum collapses, as your arms are binded, so you can protect you head and neck when it collapses, so there is a very high risk in that area. Also there is a very high risk when it comes to your archillies tendon will be put under alot of pressure, which can lead to it snapping. The risks are the same in both professional rugby in terms of scrums as it is in amatur, the difference is the ability of the players and the size of the players, so the damage that could occur with a 900kg pack, is probably less then that of a 600kg pack, but of course conditioning of professional props will be different to that of amatur, so they can take higher damage.
2. As mentioned either Scrum half or Fly Half you are less likely to get big hits, however because of the usual size of a scrum half, I personally taget them and put them under alot of pressure. Because of their size they can be likely to get hurt, however their position means that not alot of momentum can go into the tackle more often then not. At fly half, you run the risk of someone lining you up, especially if the scrum half's passes are slow and inaccurate. However they will have more time with the ball to get rid of it.
3. Tough question. Both the front row and second row have the archillies heel risk when in a scrum, and also neck injuries. However I don't know if I'd say there is a position in which long injuries will occur more then in other positions. Alot of things come down to conditioning, which may differ depending on the position, however because 50% of injuries involving tackling occur while tackling, any position is at risk. I'd say one good thing about being in a forward is that fact you often have to be big to begin with. When a prop hits a winger as hard as he can, injuries will occur.
4. Well, in theory not really. However as I mentioned, with the size and strength of professional rugby players, obviously the impact will be greater, despite the conditioning of the players.