1. This one is up to you. You've only leveled 2 classes, so it's not a huge loss for you to erase and make a new main. Alternatively, you can keep your current character and just make an alt. You can also spend 30 min to simply watch the cinematics in your inn room as well. It's not a complete reset, but it may jog your memory enough on what's been happening.

2. Tanks calculate damage based off STR and VIT (i don't know the exact ratio). The basic point however, is that tanks should get more benefit from accessories of Fending with VIT materia than from Strength accessories with STR materia.

3. Considering you've already played Heavensward, no. There has been no universal boost in level speed or exp gain like they did for pre-50s at Heavensward launch. It is however easier to gear up to i240, but considering that you are sitting comfortably at i250, this doesn't apply to you.