Not a bad list Toast, I like the nurgle/slaanesh combo as well (necrophilia
). As Unforgiven notes, you can't take 3rd Eye twice ... I'd say take it only on the slaanesh guy, as the lore of nurgle is HOT and the lore of slaanesh is pointless against ItP armies, so stealing their spells would help him have some more options.
Personally I think your lord is kinda bloaty - the word of agony and the demonic horse mostly. The word is pretty awesome, but dropping the word and downgrading the demo-horse to a barded steed (+2 armor save, for a rocking 1+) gets you 66 points. Add 25 for the dropped Eye + 4 extra and you've got 95 points to buy more maniacs with (!!).
What to add? I'd say a nurgle spawn (85 points) to guard your rear / flanks and generally do crazy things. T5 and -1 to hit makes him a tough cookie and spawn are always fun. I'd really like to see the sorcerers on horses (2+ save), but that's 32 points total and foot sorcerer models are easier to find in general.
On the marauder cav I think you've got the build spot on, with
MoS making the ultimate little light cav baiters, flankers and war machine hunters.
Oh, and props on the combo on the lord, glaive + amulet is a neat trick
- Salvage
EDIT: Also, the lord totally gets to benefit from the frenzy if he's riding with the knights. 6 attaks with the glaive, sexy.
EDIT2: I'm not sure what to do with the 10 points left over after the spawn. Maybe combine with the spell familiar on the slaanesh sorcerer to get him a nice 25 point item? Not sure what that would be (power stone?), but another so-so slaanesh spell doesn't seem all that great. Or you could give the lord stream of corruption or another <25 point gift for flavor? OR go with conjoined homunculus on the nurgle sorcerer, that's the other hot gift you see a lot of. Might go stupid but big payoff.