Things are quiet on the thread at the moment, so here's one way that Johnson could resolve the conflict between obeying the Benn bill and leaving on 31st October.

1. He writes the letter (making plain he is under duress, probably)

2. He persuades the EU to offer any other date for the extension than the default in the bill. Doesn't matter what.

3. Obeying the bill, he has to bring the changed date back for parliamentary approval

4. ..And gets the house to reject that date. Then he is free to 'no-deal' having obeyed the law at all points.