Sæll, Llama.
I don't really know the answers to your
latest questions. Yes, I think "láta + pp."
may be somewhat archaic/poetic in the way
it's used here:
"Ok Ellu bak,
at, lét, hinns sat,
Ívarr, ara,
Jórvík, skorit."
But note the difference in meaning
Hann lét bakit skorit. = He had the back cut.
Hann lét skera bakit. = He had SOMEONE cut the back.
We still use "láta + pp." in various contexts, for example:
Hún lét það ósagt. = She refrained from saying it.
Hann lét það ógert. = He refrained from doing it.
Kveðja,
Haukur