Hello,
I wish I could be more help, but I do not play slaanesh and am relatively new to aos. I do however run a khorne army with mixed slaves to darkness, and have purchased the slaves to darkness “godsworn warband” box set. While I do not regret the purchase (there are tons of great models) I find that I only really use three units from the box. Slaves are a great army, they are just in a rough spot at the moment and just lack any punch to compete with most other armies. They will be receiving a battletome sometime in the foreseeable future, so if you are willing to wait for that, it could be a good purchase. Currently you are better off running them as part of a slaanesh warband. Some units to consider:
The chaos warshrine is undoubtedly one of the best models in the box. Its invulnerable bubble has been a huge asset in most games i've taken it.
The chaos knights are a good cavalry unit, I especially get use out of them as khorne is a relatively slower army. They may not be as good for slaanesh as you have access to better faster units. They are most useful for capturing objectives and guarding them ive found. Their damage output on the other hand is lacking, this is a common theme for much of the slaves to darkness army.
Lastly are chaos warriors, these guys are slower chaos knights essentially. A good sturdy unit with 2 wounds a piece and a +4 save, plus a MW save. Add in the warshrines buff and you have a unit with decent staying power. These guys can be good for objective capturing, as well as screening your opponent. They can make a good battle line choice, as some slaanesh options tend to be squishy.
Unfortunately the other units I find will underperform especially for the points cost. For slaanesh, you might be better off purchasing a box of warriors and a warshrine separately as this will be cheaper in the short term. Something to consider is the warshrine alone cost roughly $60, while you can find the godsworn warband for about $140 from most third party retailers. The warshrine alone is a little under half the box value, if you are like me and love the look of the entire model range, the godsworn warband will be the cheapest option for all of the units you get.
STD is good at filling in certain weaknesses in your army, for me I get access to buffs and faster units. You should carefully consider you list and see what it is lacking. If your lucky STD will have a unit that can cover the gaps in your list. All of this is based on personal experience, I am in no way an expert, but this is what I wish I knew before I bought the godsworn warband box. I wish I could help you with your other questions, but hopefully this answers one of them.