lyok

Correct, the pre-incubation was to help remove antibodies that will bind to proteins other than MBP that are present in the lysate.  The anti-serum in question has not been further purified to produce purified antibodies against MBP, if it is the one I found at NEB (cat number E8030S), it would say "Affinity purified" if it were.

An anti-serum is different to an antibody.  Anti-sera are where the blood has been collected from the immunized animal and the red blood cells removed, but nothing else.  An antibody is a small component of the anti-serum that binds specifically to a particular sequence.  In all anti-sera, there are many many many antibodies against all sorts of things from food the animal is eating, bacteria, viruses, fungi, dust... you name it, it's there.  A typical antibody recognition sequence is less than 15 amino-acids in length, often as few as 4 aa's, which as you can imagine, is not very specific.  This is the reason that either further purification is needed, or some sort of pre-incubation to help remove antibodies that bind to proteins other than the one you are looking for.

Yes.

This is a measurement of how much product is formed by an enzyme in a given amount of time per mg of enzyme .  I use specific activity for kinase reactions and what this means is that my kinase is capable of transferring X umol phosphate onto a substrate in a certain amount of time given a certain amount of active kinase.  Usually a pure enzyme gives a constant number so this is also a good measurement of an enzyme's purity.