Total Volume for Centaur: ~1200 metric tons (NASA says ~900)
Total Volume for Shepherding Craft: ~ 500 metric tons (NASA says ~350)
Chemical analysis should lie within the range for comets.
("Water Vapor" in the chart is actually detected as OH)
They will detect >50% OH, however direct
evidence for H2O should amount to <10%.
detect a surprising amount of Na (sodium), perhaps up to 20%
More fine, talcum powder like dust than expected.
4 days past full would put the Moon near the outer edge of the magnetotail, I think. If the probes happen to move in and out of the magnetotail, that might affect charge balance. Depending on strength, direction and flux of the IMF, and charge state of the probes, there might be a weak discharge, possibly detected as a faint/weak "whistler", probably not a visible flash though. Although the impact will be about 1/3 more energetic than predicted, the crater won't be 1/3 larger, more like 10% larger. I wouldn't be surprised to see a dim, or dark, aurora like ionization/polarization of the dust and gas plume shortly after impact. I would also expect organized, vortical "polarization" of the dust around the polar magnetic field (weak as it may be) There should be an electrostatic dust layer that will eventually form a few meters over the surface, and I wouldn't be surprised if it mostly takes the shape of a broken ring around the southern pole.
I predict a few surprises for NASA, at least.
I wonder if Wal Thronhill will have a prediction for this one. He nailed deep impact, although these moon impacts should be far less dramatic.