[Davros]

Went back and reloaded the game at Helm's hold. The trap wouldn't take damage form a magic arrow, so I had to trigger it myself (barely survived). The lock is a level 34 - I couldn't quite pick it at that stage with a +3 set of thieves tools (I was a 29 - some +6 stuff and I would have done it) - the chest would take no physical damage. A rogue could give you an answer - anyone else (even my half rogue) won't know - the chest was also resistant to the knock spell.