[dcb]

re: longer injector times at idle. It may be that you have to use the other injector lead too. Or that somehow we are looking at the low side of the pulse. But it could be that it is positively switched also.

You might try changing the code for a positively switched injector and see if that helps:
i.e. change
attachInterrupt(0, processInjOpen, FALLING);
attachInterrupt(1, processInjClosed, RISING);

to
attachInterrupt(0, processInjOpen, RISING);
attachInterrupt(1, processInjClosed, FALLING);