substitution

Hello, the league for my league OHTSL say "either team may substitute at any stoppage of play (this is FIFA)" However, does this mean blue team can substitute on a red throw even if red team isn't substituting? This could be used to stop the momentum of an attacking team, as a referee should I...

A player has to leave the field of play after injury treatment. He may only re-enter after play has restarted, is what the Laws of the Game say now. But may he be substituted? Or does the substitution have to wait because re-entering was not allowed yet?