Reason for the seperate supply arrangement i.e. one for the micro and one for the ignitor is that ignition systems are very noisy and can cause many headaches.
By isolating the 2 systems can sometimes be an advantage.
Don't know if I like the "high" side switching arrangement you have with the npn opto either.
Others may care to comment but in my opinion, as the micro starts to turn the opto transistor on, the emitter voltage starts to rise due to it's load circuit, turning the transistor off again.
Might be better to look for a logic level mosfet perhaps driven by a slightly different circuit around the opto isolator.
As I say, others more qualified may care to comment there.