A few more thoughts on "Judy" (even though I agree there's likely no clear answer, and this is just spinning wheels):
When Mr. C. visits Jeffries in Pt. 15, Jeffries tells him "you've already met Judy." Assuming that Jeffries isn't bullshitting (which he may be; he then apparently gives Mr. C. fake coordinates, matching Ray's, both leading to the "shock rock") -- what would that mean? Who has Mr. C. and/or Cooper already met, who would qualify as Judy? Probably not the "Experiment" (unless she popped out of the glass box for a drink w/Mr. C. at some point). The "real Dale" has met Sarah in the past; so I guess if Judy is possessing Sarah, that may work out.
Back in Pt. 2, Mr. C. tells Darya that what he "wants" is the strange, inky symbol seen on the playing card. That symbol also appears on Hawk's map, and Hawk says "Don't even ask about that" (or whatever). If we take that to be a symbol for Judy (which feels as plausible as anything else), I take Judy to be something of a local phenomenon -- tied in with the Lodge entrance, the portal to the Fireman, and everything else. After all, Briggs was assigned to hang around Twin Peaks because of its links with the paranormal (and specifically to search for the White Lodge); and before he disappeared, he told Gordon and Cooper that he had discovered the Judy entity. Of course, Jeffries was also aware of Judy, back in the '80s; so that complicates things a bit.
Whatever is possessing Sarah does not seem like an "extreme negative force," or something that would be represented by a fearsome-looking symbol on an ancient map -- more like a minor Lodge spirit, limited in its powers, that takes over a grieving mother and kills a barfly. I also don't think that this "spirit" is the same thing as the Experiment: darting around to NM, NYC, etc. I do acknowledge, however, that the Experiment (as seen in the bomb blast) seems to have little horns, and resembles the inky symbol
that Mr. C. is looking for.
At some point, I'll watch the whole thing again, and maybe more will fall into place on a second viewing (...or not).
Btw -- here's an interesting detail I forgot from the original series finale (as summarized at the Twin Peaks Wiki site
Nearby, Major Briggs and his wife are at a table, happy to be back together. Jacoby arrives with a dazed-looking Sarah Palmer, who delivers a message to Major Briggs, recanting in a garbled voice: "I'm in the Black Lodge with Dale Cooper. I'm waiting for you."