Q1 The three wire connection works because there is a voltage between each of the three "line" connections A, B and C.
To represent the generators with + and - symbols seems a bit silly to me, there is nothing DC about it.
There are three AC sources, the voltages are the same but they are 120degrees out of phase with each other.
When connected in the star configuration, the line voltages are SQRT(3) x the phase voltages.
Q2 When connected in delta configuration, summing the voltages around the loop of three generators, you end up with zero which is why you are not shorting out the three generators.
Coming back to the idea of three lines and no neutral, for a star connected generator and a star connected load, if the load is balanced, there is no current flowing in the neutral anyway, so you dont need it. (This is not true for an unbalanced load, there will be a current in the neutral, and if the neutral is disconnected, the star point of the load will be at some voltage, not zero as with a balance load.)
JimB