I know you asked for "help with theme and symbolism (including
allusions)" in Sweeney, but my suggestion is in another direction: read
and reread the poem . If you are like me, it will begin to intrude on
you -- pieces of it will make sense, or new sense; you'll feel the
emotions, whether or not you understand theme and symbolism. The
surface of any poem can be compelling for a long time, thought too often
it is given short-shrift.
What about the way "Sweeney Among the Nightingales" is written?
Robert Frost wrote to a friend: "Technique becomes as much material as
material itself." It's been a while since I've suggested Paul Fussell's
_Poetic Meter and Poetic Form_, so you won't be the only one who, I
hope, gets the pleasure of reading it for the first time.