I think I read Chillrend is connected to a Thieves Guild quest. I don't want to do the questline with my main character but I want the sword. Is this possible?

You can't enter the house without doing the questline even if it should be entirely possible, so no.

you can jump on the brazier in front of the temple, then jump to the roof of mercers, drop to the door, pick the door and enter.

you can also fly directly up, if you have a shield, bucket and telekinesis.

That's awesome! Thank you!!