This ain't the development league. Devers is 20 in his first MLB and not even his rookie year. This is September, not April.
The 8 dingers are both real and significant,just as you say. But they were all in his first 20 games. In the next 16 he has 0 dingers and 2 doubles. Plus we are all watching his every at bat, which tell me that he has yet to adjust to what opposing pitchers are throwing.
You make a good point that leaving Devers in the lineup, even when he isn't hitting, won't make that much difference given the rest of the lineup, but you are also forgetting that his defense ain't helping either. He has 7 errors in 34 games and a DWAR of 0.
For all those reasons--pennant race, Yankees on our tail, 23 games left, struggles at the plate, age 20 in first MLB season, doubtful defense--I give Farrell on pass on whatever he does. I honestly think Devers will get back in there precisely because of that stupendous hitting talent. But I am not quite ready to throw Farrell into managerial purgatory because he is mishandling Devers.