Can you Play as the Pope?!
As you are technically playing a dynasty in CK2, governments that don't work on dynastic grounds are generally not playable. Which basically means you can't play as non-merchant Republics nor as Theocracies. So no, you can't play as the Pope.
However, you can have a certain control on Papal election if you have Sons of Abraham should you want to. What you need to do is make sure your candidate gets elected. Alternatively, you can always crown an Antipope and go for St. Peter's throne to put make him the real Pope...
There are also a few mods that allow you to play as the Papacy. Don't have their names but I think they should be easy to find on Steam or the Paradox forum.
On the line of crusading, can the Pope Target ANY territory? so say, Tunisia and the Western Mediterranean?
Yes he can. In practice, the IA is coded to prioritize certain targets over others, such as the Holy Sites of Catholicism (which includes Jerusalem) or parts of Western Europe that should be Christianized. So Tunisia and the Western Mediteranean aren't necessarilly among the top priorities, but the Pope can target them.
However, with the way Crusades work now, you can have an influence on the target of the Crusade should you wish to: it's not going to be free of course and it's not going to be without potential negative consequences but you can.
Sorry for almost spamming the Thread but I had a brilliant idea. Could you Alexander the Great the Middle East using the ERE? Has anyone done that?
I believe that's one of the game's achievements but I might be wrong.
Otherwise, you can bet your ass someone tried that. Hell, considering there is an event that gives you Alexander's bloodline and that you can also play as Hellenic Pagans, you probably can even become Alexander 2.0