It is very challenging to play without a lower arm, as rugby is a lot about passing and catching a ball. In the Dutch ladies league I know of a girl playing with a same disability. On the lower levels it is not easy but you can and are respected for it. On the higher levels, the pace of the game is just too high to keep up with a lower arm missing.
The guy at the bench was indeed getting some examination by a medic. He was indeed injured. He had some collarbone surgery this week, I think I heared he suffered 3 fractures in his collarbone...
Game can be tough. The town of my rugby club(s) has now had 3 weeks in a row that the ambulance was called to the pitch for expected/ potential neck or spine injuries, but in all of the cases it turned out 'nothing was wrong'. But in rugby people are aware of the potential danger and if there is any doubt with neck or spines, the ambulance is called.
On the real professional level, there are doctors watching the game on different screens and can even demand a player to be taken of the pitch for a so-called 'head injury assessment' mostly concussion risk. Also the referee can decide to send someone of to have him or her checked for concussions, although usually the medical staff of the teams take their responsibility as well.