What is the benefit of a happy player over a neutral player? Also, I had a 54/96 rg who was unhappy cuz he wasn't starting the first 3 games. If i start him now and continue starting him, would he eventually become neutral (and how long would it take for that to happen?). Thanks!

Unhappy players won't sign new contract extensions with you. Happy players just take longer to drop to unhappy status if they don't feel they're getting sufficient playtime at their preferred position. There is zero downside other than whether they'll sign a contract with you though.

Your guard should be plenty happy by the end of the season if he's starting.