Rotating LB's is difficult on a play by play basis, especially if the opposing Offense is going uptempo no huddle, but try to do it by series.
Start off 1st series with the primary starters & then rotate between possessions with the backups with guys like Bonner, Marcelin, Pruitt etc..
You want the faster LB's in on passing downs & the Old vets in on run downs. Easier said than done though, because teams will recognize personnel on certain play calls & purposely go no huddle to catch them off guard.
Poor tackling is effort & ability.
Fatigue only plays a factor as it pertains to the "when", during a game. If you're seeing missed tackles late in games, then yeah fatigue is apart of it. But missed tackles in general have to do with play recognition, angles & beating your man to the spot/ball.
When you know what the play is going to be you can get a head start on the Offense & beat your matchup to the ball. Once you meet at the point of attack, use the Eagle & Boa concept.
Arms wide Eagle to enclose the ball carrier/WR/QB & then once you make contact with the runner, Boa to wrap him up.