Personally, I love flayed skull, I will probably always use that. Black Heart is not bad though, only issue is it doesn't really benefit warriors at all, while all the other traits do. Also at such a low pts level you're not going to get much use out of agents of vect, the faq changed agents to cost 4cp, can't even have that many with 2 patrols at 500pts. Depends on your playstyle though really, I know many players love black heart because of the +6 save for vehicles and the writ of living muse relic, I prefer flayed skull though because it helps buff all your units, vehicles and warriors and it's a more offensive than defensive focused trait.
For cult, I don't think you can go wrong with using any of those traits. Cursed blade is probably the best for wyches, red grief is definetly best for reavers, while cult of strife is sorta in the middle and benefits all the wych units. Personally I use red grief because I tend to use lots of reavers, but any of the traits works well.
Disintegrator cannons and dark lances are both solid choices. However, disintegrators are more variable as they work against a wider variety of targets, and are really good for dealing with pesky units like terminator equivalents given they have lots of shots and 2 damage. They are even pretty solid against vehicles, wounding on 5s isn't that big of an issue when you have say 24 disintegrator cannon shots in a higher pts list. They also tend to be more consistent damage wise, the d6 damage on dark lances is nice but it means sometimes they do great and deal 6 damage, sometimes just 1. Dissies can always be counted on to do a solid 2 damage. All that being said, I wouldn't just ignore dark lances, my current build is to take 2 dissies ravagers and 1 with dark lances.
The Night is Dark and Full of Terrors...