The analyses of a text #5

The narrator recalls his adolescence.
After the funeral of his old nurse Peggotty's husband, he visited Peggotty's elder brother Mr. Peggotty's house to dinner.
Mr. Peggotty's nephew, adapted son Ham was due to come with his niece, Ham's fiance Em'ly but Ham came to alone.
Em'ly fled with the narrator' best friend Steerforth with leaving a letter to Ham.
The narrator was taking a walk the next day morning.

Here "them" refers to Mr. Peggotty and Ham.

.......................
It was on the beach, close down by the sea, that I found them. It would have been easy to perceive that they had not slept all last night, even if Peggotty had failed to tell me of their still sitting just as I left them, when it was broad day. They looked worn; and I thought Mr. Peggotty's head was bowed in one night more than in all the years I had known him. But they were both as grave and steady as the sea itself, then lying beneath a dark sky, waveless - yet with a heavy roll upon it, as if it breathed in its rest - and touched, on the horizon, with a strip of silvery light from the unseen sun.
[David Copperfield by Charles Dickens]
1. I'd like to know "then lying beneath a dark sky" is the abbreviation of "and then lying beneath a dark sky."
2. I think the subject of "touched" is "it"
So I was wondering why there is a dash before "and."
Thank you in advance for your help.