"what do you do on a 2-3 zone, at SF-PF, go for blocks or go for speed?"
In your SF? Both. I think ideally, you want a SF that meets all of your needs in the ATH/SPD/REB/DEF/BH/PASS departments, but still has high enough SB that it wont negate the skills of the PF.
"My best SB ever was a C on a man to man team. Did he disrupt only his opponent shot or did he have an impact on the whole team. To me, the answer aren't clear at all."
I would think in a M2M scenario, Your C's blocking *ABILITY* would have only factored in the equation of defending the opp. C's shots. However, since the question "who gets credit for the block?" is decided after "does the shot go in?", it is possible that your C recieved credit for more blocks that his abiltity directly impacted.
"I know it works on FG% but how does it affect it,"
SB is one part of a player's defensive ability, along with DEF/ATH/and a dash of SPD. The results are pitted against the results of your opposing players offensive abilties. The magical RNG dice are rolled & this answers the question "does the shot go in?"
does it affect post players... if so, can it affect cutting players or penetration plays?
Yes. Yes, & Yes.