Shield projection causes the guardian to absorb some of the damage taken by nearby allies, diverting it to its own massive shield reserves.  It gets some minor damage reduction, but otherwise that's what it does: juggles who is taking how much damage.  On its own this will quickly deplete your guardians of shields.  What you do is combo it up with a Progenitor.  With several guardians absorbing damage from your whole fleet, shield restore can provide you a massive amount of extra tanking power.  It's not uncommon to see an Advent fleet with sufficient coverage simply not taking any damage at all.

The basic idea is to form the Advent Battleball with Progen+Halcyon+Guardians+Illuminators, with the Illuminators dealing the damage, Guardians using Shield Projection to absorb as much damage as they can from the various targets within your battleball, as well as using repulse to force away all those annoying Heavy Cruisers that keep trying to molest your precious illums, and the Progen using Shield Restore to keep the Guardians' shields up (thus allowing them to use Shield Projection more and keeping your fleet alive). I'd suggest using half for shield restore and half for repulse since they use a good load of antimatter, and disciples tend to be ripped to shreds if they get close enough to use their special ability. The Halcyon is obviously there to keep those pesky strikecraft away from your valuable illums.

To give you and idea of just how powerful this synergy is, it was (and still probably is) capable of tanking through the Marza's Missile Barrage with most of your illums surviving to fight on.

1. I'm not sure on this one but I don't think the damage is split for each guardian. In any case you should generally only be having one or two firing shield projection at a time so you don't run down on antimatter too quickly and give your guardians time to recover their shields (using the Progen as and when a number of them are down to low shields/health)

2. Yeah they do, but since the illums outrange Assailants (I think, though I may be confusing this with Repulse), and the Battleball can move, it makes little difference in the long run.

3. I generally go for 6/7:1 but then I tend to fight in a very offense-heavy way and neglect my support cruisers a bit.

Volt - the shield projection does not stack.  In other words, if multiple shield guardians fire their shield projectors and the shield projectors overlap each other, it's wasted.  So I tend to manually stagger it.

If they are focusing on 1 target, the battleball tactic doesnt do much (and shouldnt against a whole fleet focused on 1 ship), but you can still see how a single ship (which happens to be the target) takes 33% less damage+has its shields regenerated via the mothership. that can add up to a significant increase in life. you could add in the repulse to buy yourself a little extra regen time as well for that single target. but where its most effective is when the opponent targets your whole fleet. the damage is already spread out, and you're affecting your entire fleet's shields.

There are other significant ships to help out this configuration, but which really depend on style and resources. the mothership, the gaurdian and the illuminators are key ships in nearly all advent fleets however (as i said there are other ships. you dont just spam a combination of these only). advent uses alot of ships that alone dont work too well, but when mixed with specific other ships and tactics, synergize very well.

On the otherhand, you have that backwards. the gaurdians dont help the SR work better. the SR helps the gaurdians work better. try this experiment. take a fleet of ships that include the mothership, at least 5 gaurdians, a lot of illuminators and whatever else you like to even out that fleet. save the game before you get into a nice matchup against the AI or a willing participant in this experiment. take note of how many ships you lost, how long the battle was, whether you won or lost (obviously this shows a bit better if you win here though). then reload the save and do the battle again but without the gaurdians.