Is Cersei overstepping the bounds of royal authority by preventing the Redwyne fleet from returning home in defense of the Reach?

It’s one of those circumstances where she’s violating the spirit of the law while remaining just within the letter of the law: with the War of Five Kings technically still on, she can demand military support from the King’s subjects. However, by refusing to let them return home home to defend the Reach, she’s getting close to the level of Aenys or Aerys I in terms of not holding up the king’s obligation to protect his subjects.