The advantage only lasts for a maximum of three seconds

Garbage. You just made that up.

has to make a split decision of whether Porro or Grealish is going to get the ball.

No he doesn’t. He can wait to see how it pans out to see if it is an advantage or not.

If he plays on, and Porro gets it, enough time has probably passed to not bring it back.

No, the time does not matter. Grealish being through on goal was the result of the action which was being assessed as to if it results in an advantage or not. That is when you make the decision, not before.