Im being asked to go to Llethan Manor and check for evidence about an assassination plot. i have a feeling things will go messy there and i know thats a hlaalu place and i dont want to do the quest if it will get me expelled from house hlaalu or anything like that. i know those set of quests are optional and not related to the MQ of the DLC, but still is it worth doing them?

Also can anyone give me a brief explanation of whats going on in Mournhold? without spoilers, just so i can know where to stand. im not used to Morrowind's gameplay that much and feeling a bit lost with this whole DLC to be honest.

Even though Llethan Manor houses a Hlaalu family, they aren't actually part of that faction, so you're safe. And you can easily get through it without pissing off the inhabitants, if you're appropriately cautious.

Are you looking for a quick overview of what's going on in Mournhold?

Almalexia and King Helseth are having a petty power struggle because the Temple wants to have a weak king it can control and Helseth is no weak king. You have to pick a side.