1) Can i sue spellstrike as attack of opportunity?
At 2nd level, whenever a magus casts a spell with a range of “touch” from the magus spell list, he can deliver the spell through any weapon he is wielding as part of a melee attack.
Since you can't cast a spell during an attack of opportunity, I would have to say no.
2) I choose to fight defensively on my turn. I take the CA bonus even before the attack or only after the attack? I ask this in case of attacks of opportunity before my attack.
You can choose to fight defensively when attacking. If you do so, you take a –4 penalty on all attacks in a round to gain a +2 to AC until the start of your next turn.
You gain both the -4 to hit and the +2 to AC at the same moment. Think of it as taking a fighting stance. Instead of being in a free-form stance that allows for easy defense and easy offense, you concentrate more on defense than offense.
Fighting Defensively is a Standard action, so it can be taken at anytime that one could take a Standard Action. However, if, during movement and you haven't taken your Standard Action yet, you provoke an attack of opportunity, you do not gain the benefits of Fighting Defensively as you haven't taken your Standard Action yet. You must take your Standard Action before your Move Action to gain the benefits of Fighting Defensively.
