Sorry about the late replies, but I haven't been checking this forum for some time, as I've decided to no longer support my mods here (I'm not stopping development - just not posting here any more).
To answer a couple of questions:
The themes_id error means you probably deleted the subSilver theme. You should change the default themes_id in the im_prefs table to something else, probably... this was fixed in a later version.
Why is it possible to send an im to someone not online? Because they can receive the message when they do come online.
I think it is right that the IM window must be open to receive messages. You can only receive messages when a page checks for messages (meaning, when a page loads). So, if you didn't need to have the IM window open, then you could only receive messages when you go from one page on the forum to another. You don't have to let the popup open automatically... the Launch IM link will change to "You have x new messages" to tell someone they have a message.
Now, for some status updates in regard to this mod. First, I've changed the name to Prillian. Second, there's a couple of spin off mods now: Prillian Lite (basically a watered-down phpBB IM) and Buddy/Ignore/Disallow List. The latter extends this mod's buddy/ignore functions to other parts of a forum.