Defense Fields: I'm sitting here watching the ramp-up to the Reliant's first strike...DVD-ROMs are great for this! Anyway, when the circle is going around the saucer, I've always taken it (and still do take it this way) that when the command of "Energize Defense Fields" goes out, that meant to shunt power to weapons and shields, giving them power to fire when necessary. The circle goes around where the phaser banks are located, no further. This tells me that the phasers are receiving primary power, as also the buttons for charging weapons are being pressed (Warp power transfer is the console there) and shields are also getting power...but all of these systems are on standby. None are activated because, hey, why start firing on a Federation starship without just cause? And why raise shields for the same reason? So power is readied to those systems, but can't be continued due to the first-strike phaser volley into Engineering.
Swing-around torpedo strike: From the angle that the torpedo is fired at, the trajectory it takes and the place where it detonates, the only possible impact point would be somewhere on the starboard half of the upper saucer, probably close to the bridge where the saucer starts angling upwards. This kind of detonation close to the bridge is going to set all sorts of fires around its area, and is more than likely going to put a bunch of energy into feedback loops through the circuits it touches. Remember, a torpedo is inside of a casing that has both matter and antimatter which are kept separate until point of detonation...they're released, rushing into each other and creating explosive energy...
...which makes the torpedoes in ST III take a completely different approach. Both sides appeared to have some sort of effect that resembles an ion cannon effect, not doing any damage but knocking out systems instead. *shrugs* It's the same disparity issue I have with that and the speeds at which "full impulse" seem to vary greatly. In ST III, it's kind of slow. In ST VI, it's a blasting forward of incredible speed...which is actually what it should be, given that full impulse (I believe) is supposed to be about .8 light speed.
Just my input.