1. Think of it like this:
ELAC makes sure that the aircraft will not go BELOW V.a.prot/max. It is a minimum speed, not really a target speed as such. It will pitch down just enough to maintain the minimum airspeed. No more, no less. It is not a fixed value, and depends on lots of factors.
Say you are climbing really steeply, and the autothrust is already at Climb power, but you are still decelerating. When you reach V.a.prot/max the ELAC will not allow you to climb that steeply, so will lower the nose so you don't go below and stall.
2. EIU (Engine Interface Unit) is the bit that interfaces between the FMGS and the FADEC. Not sure exactly what that means... I have a picture of the architecture but can't figure out how to post it on here. If you can help, I will post it!
3. By vertical trajectory it means a certain climb rate or flight path angle.
Just briefly (I have to go to work, will be back on Jan 1st)
Set yourself up in the simulator, normal flight, Athr On, FL300. Select speed.
Select FL100, pull for open descent. You will see Athr commands THR IDLE. Select a high speed, watch as the pitch increases, descent rate increases, then reduces as the speed is reached. Then decrease selected speed (say by 20kt) and watch as it increases pitch, reduces descent rate, to decelerate, then steepens up the pitch once the speed is reached.
Now turn off autothrust, set a low thrust value (say 700kg.hr per engine). Do the same. It will pitch to maintain the speed. Thrust is fixed, so it will give whatever pitch is required to maintain the speed.
Now re engage autothrust, and select vertical speed -1000fpm. Again increase and decrease the selected airspeed, and watch how the autothrust compensates to maintain it this time, not the pitch. This is because the FD is controlling the VERTICAL TRAJECTORY.
When you are done, set up a climb and do the same exercises (using climb thrust if Athr is not on) and see how it works there - pretty much the same.
That should give you a good understanding.
Sorry I won't be around for next couple of days!