A 140 The only place it's really talked about is on PHB 15, where the trance is described as the elf dreaming, which in my mind sounds as restrictive as sleep does. Sleep is in general poorly defined in the core rules, though it should be noted that the Helpless condition lists 'sleeping' as one of the ways that it is in effect and what little rules provided for the Elven trance doesn't seem to change anything besides how long it takes to be fully refreshed, so an elf trancing would be just as Helpless as anyone else sleeping.
Q 142 Would a Merciful weapon convert the Vicious property's damage to nonlethal as well? Would it effect the backlash damage for using one, since it's 'damage the weapon deals'? How does Merciful interact with energy properties, like Flaming or Frost?