a), ILLEGAL; an absent player may not return to the lineup. In b), LEGAL (provided the player has a re-entry remaining); an exception exists if an the Absent Player LEFT THE GAME UNDER THE BLOOD RULE (5.2.A.1-2, p 30)
I really hate this exception for one specific reason. While I agree with both premises, It is when they BOTH exist that you have a potential issue. If a player was removed because of the INJURY (a sprained ankle) and there was BLEEDING INVOLVED, would she be considered to have been removed due to the injury or under the blood rule? It seems that if you get hurt, it is better to make sure you draw blood, just saying.....