Yes, A 15th level swashbuckler fighting defensively as a swift action via dizzying defense can also make a full attack, getting a total of 4 attacks (all at -2 to hit, +4 to AC)
Or anything else you can normally do. This ability grants 1 extra attack per round, but you have to fight defensively. It is just that simple. It may be a bit OP, but still, simple. And still not as good as Parry and Riposte, Precise Strike, Subtle Blade, Superior Feint, Perfect Thrust, or the archetype deed Inspired Strike.
The other issue to be aware of is that fighting defensively affects all attacks in the round. Therefore if you full attack and THEN fight defensively, you will change your to-hits with attacks that have already happened. Therefore, I would rule that you must use the ability first, so as not to mess things up.
Speaking of Perfect Thrust, you will be making only two attacks at level 15. But with 4 rolls. Your second attack being Perfect Thrust, which for ZERO Panache, allows you to attack touch AC and IGNORE ALL DR!!! At that level, everything has DR.
As far as why the class is so OP, I think it was designed with Dex-to-hit in mind, which would limit Str damage, but they forgot to add something to force you to use Dex-to-hit.
The benefit of the feat Bodyguard is used after the foe's attack roll is made but before the GM reveals the attack roll's result
Combat on Combat Statistics on Attack Roll says
An attack roll represents your attempt to strike your opponent on your turn in a round. When you make an attack roll, you roll a d20 and add your attack bonus. (Other modifiers may also apply to this roll.) If your result equals or beats the target’s Armor Class, you hit and deal damage.
You, in this case, is the monster attacking your buddy. Were the monster to make the attack roll and the result revealed, everyone would suffer the consequences of the attack. It'd be already known whether or not the attack hit and deals damage, and too late to do anything about the result.
Except in rare cases, Pathfinder has no declare phase. Unlike, for example, some trading card games, a creature needn't first say that it's attacking then attack; instead, a creature just makes an attack roll. Before the monster makes the attack roll, the monster typically has a lot of other options, like making its appropriate attacks against anyone. Making the attack roll, then, is the creature declaring it's attacking its foe—by taking a poke at the dude! That is, were the benefit of the feat Bodyguard able to be used before the attack roll, it'd lock the creature into attacking that foe before the foe's locked itself into making the attack by making the attack roll!
So, typically, the only chance there is to use the benefit of the feat Bodyguard is between an already-made attack roll and the DM revealing whether or not the result means that the attack hits and deals damage.
Best Answer
Unfortunately, you can't.
Bodyguard reads:
So, it would be "No, you can't", as Bodyguard need an especific kind of action to be performed (AoO) and the Total Defense Rules states that you can't use AoO's while in Total Defense.
The rationale behind this is that while you are in Total Defense, your only concern is protecting yourself. To "be a bodyguard" can be seen as protecting someone else, so you can't really be on "Total Defense". You can look at Fighting Defensively (Scroll down to it) as an alternative to this. It won't provide the +4 but it will provide a nice +2 on your AC that can be really helpful.