You get a permanent bonus to base stats for all of your characters, current and future, by getting every class to lv.75 (does not have to be done on a single character):
Depends; Summoner's "Sub-class Growth Up" makes it so your subclass gets 50% EXP instead of just 25% EXP, so it does make things go faster, but it requires Summoner to be the main class, so if you aren't too keen on using Pets, you need to get some All-class weapons (like 12* Sigmas) to match the subclass you are using.
Yes, that is typically how it is done.
Some advice that can make leveling more bearable: save all your Tokyo Silver Bonus keys (even if they expire) until PSO2 Day, and convert them into Gold Keys in the Swap Shop; the boost from PSO2 Day will let you get more EXP in a shorter period of time as if you had been using the Silvers each day, so it saves booster time. Also, when doing Bonus Quests to level everything except Braver, grab yourself a Sigma Katana, and set Braver as your subclass no matter what mainclass you are using, so you don't have to use different weapons/mechancis just for the sake of leveling.