I always thought airbags were inflated by an electrically-fired explosive charge? Oh well, I suppose there are other options
.
Do you mean the solenoid needs to be energised for at least 200mS to operate?
Yes. You can omit Q2 and Q3 and their associated components. A 555 can drive a suitable FET directly.
It's supposed to!
Why? Airbags are use-once-only things. In an accident there would only be a single trigger. Besides, surely it won't matter if the valve opens repeatedly after the bag has deployed?
Edit:
If this is a hypothetical system, e.g. for a school project, then fine. If it's for a real-life airbag system then a lot more needs to be done to prevent any possibility of false triggering, e.g. by voltage spikes on the 12V supply.