Substitute

A substitute in the sport of cricket is a replacement player that the umpires allow when a player has been injured or become ill after the nomination of the players at the start of the game.

A substitute can act for the injured or ill player in the field, although he may not bowl, bat, or act as wicket-keeper or as captain

If a player has a runner, he may be given out if either he or his runner transgresses the rules.

What if a substitute fielder entering the field of play without permission?
The ball shall immediately become dead and the umpire shall award 5 penalty runs to the batting side

player wishing to change his shirt, boots, etc. must leave the field to do so. No substitute shall be allowed for him.

Objection to substitutes
The opposing captain shall have no right of objection to any player acting as a substitute on the field, nor as to where the substitute shall field. However, no substitute shall act as wicket-keeper.

Transgression of the Laws by a batsman who has a runner
batsman's runner is subject to the Laws. He will be regarded as a batsman except where there are specific provisions for his role as a runner
A batsman with a runner will suffer the penalty for any infringement of the Laws by his runner as though he had been himself responsible for the infringement. In particular he will be out if his runner is out "Handled the ball", "Obstructing the field" or "Run out".

What happens if both the striker and and substitute both run and reach the bowling crease ?

I dont know the answer, I think the run is not counted and rule says striker with substitute should always be on strkers end.