A) I use the sql version, so I can't help you there.
B) InventoryListener.java -> opcode == 13
Spoiler for Example:
C) I have no idea how to fix that, all the receiving player has to do is turn on his private chat, if it's the same problem I'm having that is.. the private chat will say it's online when it really isn't.
This doesn't apply to admins because their private message will show whether your private status is on or off.
D) Go visit the tutorials section there's a few I believe.
Hope this helped a bit.