What is the best number of PWM signals for Full-Bridge driving? (2 vs 4)
Why are 4 PWMs used instead of 2 to control Full-Bridge?

Phase Shifted PWM (PS-PWM) control,Space Vector Modulation (three phase)..., other kinds of switching sequence to those you have in mind.

What does the separate control of each transistor give?

HiZ control, adjustable dead time on driving side,...
Your app. can be a quasi resonant LLC converter, driven with PS-PWM.