I understand the rule which says you cannot use a leader to unify 2 or more kingdoms.
No, actually you don't. Sorry.
However, I have questions about two related scenarios.
1) Can you place a leader in a spot between two or more kingdoms? It is clear to me that this would not unite the kingdoms; but can you still place him there?
No. That's the whole point of the rule that says you cannot use a leader to unite kingdoms. Placing a leader in a spot between two kingdoms WOULD unite them, except for the fact that you can't place him there to begin with because you can't use a leader to unite kingdoms.
2) If a conflicts or disaster tile break up a single kingdom, leaving the leader as the only remaining link between two areas of the original kingdom, does this mean there are now two separate kingdoms? The answer to this would matter if one of these areas/kingdoms was attacked through external conflict. Could the leader use tiles from the areas/kingdoms on both sides of him toward that conflict?
For purposes of determining what constitutes a kingdom, a leader counts as being a special "tile." Any group of contiguous tiles that contains any leader is a kingdom, even if that leader (being a special "tile") is the only link between certain sets of those contiguous tiles.