After employing chainsaw, fire, axe, hatchet, and sledge hammer, I finally managed to remove the roots that blocked my trench for the cobblestones, and I have filled the trench with slate sand well past the second tree. I am now, finally, all set for putting the remaining cobbles from the second batch into the trench,
I also finished putting concrete slates at the edge of the trench along the left side of the lawn (which looks 'right' in the first photo, as it was taken from the far end of the lawn).
I got the slates for free a couple of years ago, and they were always meant to be used for this edge - plans just got upgraded with the cobblestones which provide a broader and nicer edge for the lawn.
I think I shall have to adjust a couple of the slates a bit, as the top edge could be a bit more straight, but all in all I am very pleased with this progress, which I had not expected to be able to make until spring next year (but the summer just keeps staying, and I may as well keep making good use of the warm dry weather, which may come to an end this week).