I don't think you can complete the stolen key quest before exposing Elrath. I don't recall the quest in great detail, but logically you can not prove or disprove his claim of loosing the key to the storage room until you have hard evidence to implicate him, at which point you can finish off the rest of the quest as well.
Once you have Elrath arrested the house gets sealed for good, and you can not get back in using Lockpick or the key.
I think a couple people have mentioned that friendly bee/wasp. There is nothing special about it, and it does not turn hostile after some trigger. An area effect spell that does not need a target may work on it (ie Hell Spikes or Poison Cloud).
You will have access to the graveyard by the church after going to the castle (which there is no reason to rush). In the latest game version I think you can Lockpick the gate (level 5) and the key for Nericon's garden will open the graveyard. There is a second Nericon garden / graveyard key in the alchemist's back room in Ars Magicana, but there is a bit of a trick needed to be able to get in there (
Stealing from Walker and
part 2).
You can head to Verdistis (north of Ars Magicana) and/or the Dark Forest (entrance near the archer's guild) before going to the castle, if you wish. There is little fighting in Verdistis (aside from the sewers) and you can find a few good traders and a couple pretty handy items there.
