Have you found the ornaments?
 

For the fashion designer on Riftlands? I managed to find 4 (3 from battles, 1 at sea), but cannot for the life of me find the last one.

Trouble is it ties to another two subquests so it's stuck on my to do list forever. Pls help.

The last one should be on a trunk just near the thief.

Yep, got that one already...the fifth still missing though. All the mobs on the island are cleared, all chests opened but no fifth pile of ornaments. Frustrating.

IIRC, 1 in a trunk, 1 at sea and 3 from battles. 3 mobs that have the ornaments are the only 3 viking mobs on Riftlands.

Ok so that's another bug. I've cleared all mobs, including the vikings, and one of them failed to give me the ornament pile. Oh wel...

Do another fly by and look for the red dots, it is possible that mob might re-spawn, I thought I killed everything, but after killing Arthur, I had couple mobs in the area that I’m 100% sure I cleared before.

I had the exact same thing - places that I absolutely know I visited and cleared out before, suddenly had new enemies to slay. One of them ended up giving me the last item.

Found it from the "cache in a tree" near where i fought the thief "Arthur Grabber"

Can anyone help me with this quest? It seems I only have 2 chests in my inventory, i think that some of them may have been disenchantable, and I might have destroyed them :S

No, you might be hitting the old bug. :( Even if you find them, they will not show up in the inventory even if it shows up in the quest log -- making it impossible to complete.

Only solution is to "cheat" using the console to get the items and/or skip it and/or restart a new saved game. :(