Shade
Monster Junkie
That looks fine to me. A constrictor snake is a probably a "2nd-level creature" so the equivalent of a summon monster IV would get you 1d4+1 of them.
Would you prefer 1d4+1 to 2d4? I'm fine either way.
I'm in favour of dropping the "Moonsea" name and adding an In The Realms underbar.
Spirit Constrictor is OK I suppose, but I would quite like to reference its original "Headless Avenger" name.
Headless Spirit Constrictor?
I'd rather not. "Headless avenger" was simply a chapter-title thingee...it was never used as an actual name for the creature. That said, I don't mind name-dropping it within the flavor text somewhere.
Updated.