Of particular note:
Q: If I have a Warlord and I use the Relics of the Thousand Sons Stratagem to give a Thousand Sons Character from a different a Sorcerous Arcana, can I select their relevant Sorcerous Arcana? A: No, as you can only select that Sorcerous Arcana if your Warlord is from that .
So this seems to state we can take the relic as long as we have a character from that Socerous Arcana.
THOUSAND SONS Q: If I manifest the Warp Reality psychic power with a unit that has the Brotherhood of Sorcerers ability, can I use that ability to select one enemy unit within 9" of the terrain feature? A: No. The Brotherhood of Sorcerers ability allows you to select a terrain feature within 24" of the Psyker.
Q: Can the ability from the Guided by the Whispers Warlord Trait be used by a model that cannot fire Overwatch due to not having a ranged weapon, not having line of sight or being out of range? A: Yes, unless another rule would prevent them from being able to fire Overwatch (such as being within 1" of enemy models or being under the effects of a rule that prevents the model from firing Overwatch).
Q: If I have a Warlord and I use the Magister Stratagem to give a Thousand Sons Character from a different a Warlord Trait, can I select their relevant Warlord Trait? A: Yes, as they are regarded as your Warlord for the purposes of that Warlord Trait