plus delay time taken from A1
It's not a delay time. Stop thinking that way. It's the gas-on-after-trigger-off interval.
and check for a trigger press during that period whilst keeping the gas on?
Something like that. Some events are independent, like pressing or releasing the trigger. Some actions happen after others. Some change the behavior of others.
Turning the trigger on should turn the gas on, and set a flag indicating that the gas is on.
Releasing the trigger should set the trigger off time.
Periodically, you see if the gas is on, the trigger is off, and the time between now and the trigger off time is greater than the gas-on-after-trigger-off interval.
You really don't know me!
I'd do the same, too, but the Arduino doesn't. It doesn't do dessert - interrupt driven or not.