Disc Riders = Mounted and Skimmer - they would need to be repriced. I would suggest a unit of 6-9 where you buy 6 as a start and have the other 3 as optionals (to give you the 9). It will help on points.
Regards Scarab Occult: I am just reading the book now so I cannot comment with any great certainty. I would ask how are the legion growing with geneseed when there is none? Are they taking on rogue librarians? If that is what the scarab Occult are, Isn't that bit much to think that there would be so many? I am all for non-Fearless and fluffy-stuff, I would just like to know it's justification.
In doing that, would there be the consideration to bring it then into line with the other Chaos lists and perhaps have mixed units of Thousand Sons Marines and Scarab Occults? Just an idea.
Keep Ahriman's Chosen as a separate formation in favour of some sort of other suggestion such as the above.
Removal of Wings from the DP (Magistar): Is this just a rules thing because the save is better on a Foot slogging DP? Taking away the wings option (although not many if any would use it) really takes away from the whole (I am going to model up an INF size) Magnus the Red which, last time I looked, had wings. Leave it in as an option, what would it matter?
First Strike: Perhaps. It does really impede on the Emperor's Children however. Could I suggest a better FF from these guys instead (as that is what I cut back from the Emperors Children list a bit), or something else perhaps (I have a mind blank at the moment) - maybe more Ignore Cover stuff?
With the Phychic talent of these guys, Strategy Rating (in there already) or even Initiative could be a good thing to play with (perhaps always count as Initiative 1+ while the Sorcerer Lord is alive)? Just somke crazy ideas to think about

I would however have a severe negative to losing the Character unit - it makes sense that the automaton unit is tied to the Character IMO. Maybe I am incorrect?
Good to see the list moving along Neal. I am reading the book atm so I will be sure to come back with other ideas as I read on.
Cheers...