Torpedo and Phaser distribution
As can be seen in the below image from 'The Best of Both Worlds Pt I', we see the Enterprise-D firing phasers from both the saucer and stardrive section:
The above image demonstrates that torpedoes can be fired from the stardrive section (it's clearly coming from below the saucer section). This is showing a front torpedo-launcher. This image shows that the stardrive section also has an aft torpedo launcher.
Looking at the below schematic, it seems that the saucer section may also be capable of launching torpedoes:
However, this is an aft-torpedo launcher, so in terms of weak spots, the saucer-section wouldn't be able to fire torpedoes from the front. However, it does have phasers there!
Memory Alpha tells us that:
Shield energies could be emitted from a localized antenna or "dish", such as a ship's navigational deflector, or from a network of "grid" emitters laid out on the object's surface, such as a starship's hull.
As the saucer section obviously lacks the main navigational deflector, I would postulate that it uses the latter i.e. a network of grid emitters. It also doesn't make much sense to use saucer separation next to a Borg Cube if the saucer lacks shielding! So, I would say they both have shielding.
In terms of power, as the saucer section lacks the matter/antimatter reactor, it wouldn't be capable of producing the same amount of energy, therefore I would expect the saucer section's shielding to not be as strong as the stardrive section's shields.
I have found no information to suggest that there is any particular advantage in terms of the stardrive section's maneuverability following saucer-separation. It stands to reason that it would be somewhat more maneuverable, considering that it doesn't have the saucer section to interfere with operations.
The Saucer section has the impulse engines ('The Best of Both Worlds Pt I'), so that would be a very maneuverable sub-light craft in its own right.
Yes, multi-vectored attack is both possible and seen. In 'The Best of Both Worlds Part 2', the Enterprise-D engages the Borg cube in such an attack with the saucer section firing antimatter streams and the stardrive section engaging it simultaneously.