Do its containers reset over time after completing the daedric quest or is it safe for storing loot?

Yes it is safe, Molag Bal gives you the house to look over it for him. It is safe to store and the bed is unowned so you may sleep in it....you just cannot have a wife/husband live there as its not technically recognized as a player home, though all containers in it are safe and considered unowned.

Ah, good to know, thanks. Maybe my character starts to use it as a secondary base. He'd be closer if his daedric ally has anything to tell.

On topic: Is there other known residences with safe containers that are not bought or obtained during guild quests?

Edit: Sorry about double post.

A few I found on the wiki, they do have some notes to be aware of (ie:some barrels are okay in one house but the chests respawn)

• http://www.uesp.net/wiki/Skyrim:Alchemist%27s_Shack, has two barrels and an endtable for containers. Also has the benefit of an alchemy lab.
  
• http://www.uesp.net/wiki/Skyrim:Meeko%27s_Shack, has a barrel and an endtable for containers. However, the barrel respawns and so do the shelves, regularly filling the house with junk.
  
• http://www.uesp.net/wiki/Skyrim:Riverside_Shack, has one safe wardrobe and a bed. See http://www.uesp.net/wiki/Skyrim:Containers#Safe_Containers for more details on safe containers.
  
• http://www.uesp.net/wiki/Skyrim:Abandoned_Shack, this is where you take part in the initiation of the dark brotherhood
  
• http://www.uesp.net/wiki/Skyrim:Traitor%27s_Post, has two wardrobes and a cupboard that you can store items in. The chest there resets along with the bandits.
  
• http://www.uesp.net/wiki/Skyrim:Lund%27s_Hut, NNW of Rorikstead. Farm house with non-functioning bed. Only the chest and end table are safe containers.

http://www.uesp.net/wiki/Skyrim:Houses

If you are on PC, there is a console command to change ownership of containers. If you don't mind using that (or even can) Drela's Cottage http://elderscrolls.wikia.com/wiki/Drelas is a great house to have, you just must defeat the mage. You can change ownership to everything storage in the place and keep it as your own.

Very useful, I'll examine those more specifically.