Deamon Prince - agreed, I'd just remove the winged option and leave the rest as-is.
GUC - I've seen suggestions about adding Disrupt to the Stream of Corruption to make up for the complete lack of Macro Weapon on any of his attack, which would certainly fit thematically, and would be more consistent with the Fester Titan and Plaguereaper (all the short-range barrages would then be 3BP, Ignore Cover, Disrupt).
However that's something that would need discussing much more widely, as a DG-specific GUC would look a bit odd. On the other hand, with LatD redux still in progress this might be a good time to do so.
Does anyone know how popular the four different Greater Daemons are in Black Legion and the various LatD lists are?
For the Walkers:
I'm coming round to the
Desecrator being fine. It initially looks much weaker than the Epic UK DG Defiler (EDIT 2 which may itself be a bit too powerful as a formation add-on), but swapping the AP5/AT6 Reaper Autocannon for the AP4/AT6 Disrupt Vomit Cannon helps a fair bit, especially in formations that have Disrupt Barrages, and 15cm move so that more than one can be in a garrisoning formation is great. Four of them with three Chaos Spawn for numbers (at the cost of having a mixed formation) for 350pt makes for a pretty tough Anti Aircraft garrison that can spread across a wide area.
EDIT 1 - your QR PDF has another error, the secondary gun should be:
Vomit Launcher 30cm AP5+/AT6+ Disrupt
Which is that one pip worse against INF.

The standard
Defiler suffers from it's usual problem of being unfocused (although taking Chaos Spawn makes them much harder to break than in other lists), and I can see them working well when added to Plague Towers due to having the same speed and a big overlap in range.
The
Plague Hulk I'm also coming round to, CC3+ with +2 EA MW on an infiltrator is vicious in more general use, I was biased due to playing against Eldar in more than half my games. Maybe swap the AP3+ Disrupt Decay Cannon for a Desecrator-style Vomit Cannon to add AT6? That way it's got some way of harming AV Skimmers outside of a single FF6+ roll.