Which Strategems available?

Hello fellow archons,
I have a questions regarding the strategems a player can use, especially, when he´s mixing factions.

Option A:
I´m just using Detachements (Spearhead and Batalion) filled with Drukhari. Easy: Only general strategems and drukhari strategems.

Option B:
I´m using a spearhead with CWE and one with Dark Eldar. Can I use the general strategems, the CWE strategems and the Dark Eldar Strategems? (With some restrictions about availability, so maybe one DE strategems is for witches only, of course).

Option C:
I´m using a big big detachement and I´m mixing dark Eldar and CWE in this detachement by using the Aeldari Keyword. In that case I can only use the general strategems since there are no aeldari strategems, right? If that´s the case, that mixing is stupid...

true, as long as you have detachments with just 1 kind of eldar there is no problem.
But if you mix 2 eldar kinds in the same detachment they suddenly lose all their superior tactics.
Thus option C is usually not the smartest thing to do (unless you also ad another detachment of a specific eldar keyword, for instance you might take a large ynnari btrigade mixed, and and two auxilarry (craftworld and dark eldar) to gain more command points and all the stratagems than if you went a battalion + spearhead + outrider.

I seem to recall GW did clarify this at some point, and Option B was the answer.
I'm trying to find it, but I'm certain that was the case.

FOUND IT.

Because it was hidden in the Codex: Death Guard FAQ.

"Q: Is it possible to use a Stratagem from Codex: Chaos Space Marines to target a unit from Codex: Death Guard?
For example, can I use the Tide of Traitors Stratagem on a unit of Cultists from a Death Guard Detachment if I have an Alpha Legion Detachment and a Death Guard Detachment in a single Battle-forged army?

A: Yes – if you have access to a Stratagem because you have an appropriate Detachment, it can be used on any permitted target: they do not need to be from that Detachment. In your example, the Alpha Legion Detachment gives access to the Chaos Space Marine Stratagems, and Tide of Traitors can be used on any Chaos Cultists – this would include any Chaos Cultists from the Death Guard Detachment."

Granted, this was specified using a Chaos example, but as a general rule would stand.

Are there any cool cwe Strategems dark eldar could use?

None of the CWE stratagems work on Aeldari units, they are all keyed to Asuryani, Craftworld or specific units.