Mel Morris is the answer to number 2, and he makes an appearance in DD (after all...he did come from there in the first place).
For your first question, if I understand correctly...you are asking how many keys can be found that you keep in your inventory when used.
Varde's Key - obtained from robert shreve just outside the forest
Ichrius's Key - created by leon shore by fusing the power of the level 3 excellector plus the 3 symbols of ichrius (wings, crown and eye)
lyns key - opens her box in ralugo
dwarve's key - opens the door to jens stensland's workshop.
copper key - one found in a secret room that drops off a tree reaper in the old garrison and one that is
silviera's key - i believe there are 14 altogether...dont ask me where they all are cuz i cant remember without playing it again for a bit.
jail key - one of these drops off a tree reaper as well in the old barracks, another in the path of poison where orladins stuff was in the secret area.
fire magic - used 12 times on lamps to unlock the palace of wind magic portal :P
silve rkey - one found in the passage to poison from ralugo in the wall where the fake prince is searching, one found in the thieves passage in a secret hallway.
mages key - one found along the shore at lake noel hidden under ground, another found on the grave of shudom in the cave beyond the cliffs.
royal key - passage of death, one in the cemetery hidden in the ground.
corridor key - 4 of these spread around the castle inside and out.
other keys that are one use:
marilyns charm - saves her kid in the battleground
lyns ring - brings the spirit of alexander to the fountain at verdite...which is awesome.
fairy fossil - use to meet guyra
orladins skull - use to bring orladins spirit back to life in the place of the beginning.
moonlight sword - needed to get the proper ending at verdite
wise fruit - used to reach intellect 5 to get all the item description unlocks and to have mark johnson tell you item truths.
kill all the golems - unlocks the cyclops event at the end of shudoms cave.
wind crystal - need to learn one wind magic to get in the palace of wind.
meet orladin - needed for the good ending of the game.
orladin's key - i forget how many, i think 18 and they are one use.