If you want your familiar to bring messages somewhere, how far can you go?

From communicating through your familiar telepathically to just posessing him and have the familiar do all the talking (provided it is able to).

Also, letters. Does the familiar have to deliver them IG in case it's killed or something like that, or can you just make a topic/send the PM?

It can, so long as it is done IG. Many people/npc's would kill most familliars on sight as a monster. As for range I'm inclined to say their range is limited by the game engine.

Actually, I've managed to smuggle my familiar down into Lower from a room in the Last Stand without being seen twice already.

The trick with getting your familiar in a different area is as follows:

Transing with your character teleports the familiar to the same area as the character. Transing with a posessed familiar keeps the character where it is and allows the familiar to keep moving. Unposessing the familiar (when it is in a different area than the character) immediately teleports it back to the character's side.

Some 'rules' on how far any seemingly nonsentical actions in the above can be exploited would be nice.

But I was more looking to find out how far communication through your familiar in the IC sense can actually go. As in, can you speak through your familiar? Should I see 'Posess familiar' as an OOC-term for transferring player control to your familiar, or in the IC sense of taking over your familiar's mind? That kind of thing.