You need the two initial gluons in your #2 because each one comes from the sea of one of the participating particles (i.e. the proton and the anti-proton). There isn't enough energy in a (anti-)proton to produce a top quark, but the energetic collision supplies enough.
The vertical line in your #3 is a space-like (anti-)quark line. It's meaning is very clear in the connection between quantum field theories and Feynman diagrams, but about the best you can say without the math is that it represents a virtual fermion. You may recall that anti-particles are mathematically equivalent to particles moving backward in time, With allows an interpretation in which it takes two interaction with the gluon to "reverse" it and then you have a single particle that comes from the in as an anti-particle, gets turned around and heads back to the future as a particle.