There is only one way I know of to get the Pope of your vassal, which is to have a King-Bishop vassal, make him Anti-Pope and press his claim on the Papacy. After this he will still be your vassal. I have tested this in version 1.06b, but getting a King-Bishop vassal is a bit of a pain in the first place. Even if your Anti-Pope is of your dynasty, pressing his claim will result in independence unless he already has a King level title (which is the same level as that of the Papacy).
I've never really tried this, but you can probably fabricate claims when Rome is not controlled by the Pope and that makes sense. You'll never get a claim on the Papacy though because it's a King level title.
Yes, you and the Dukes in Italy would have been able to vote for their favourite heir for Italy.
Generally I build cities in coastal provinces. If the province isn't coastal then I just pick castles for troops and cities for gold. The point here is that you get some very good bonuses from some of the buildings in coastal cities which make it very worthwhile. Usually you never have a reason to build a bishopric unless you are forced to due to the province not already having one.
If the county is held by a vassal of yours and you build a city then you will become mayor. You should then either assign a mayor to govern it, or just give it to your vassal Count (he will then give it to a mayor that will be his vassal). If you choose to assign a mayor and keep that mayor as your direct vassal then the Count will have a -25 opinion of you for this and will want you to transfer the vassalage. This can be ok in some situations, but you might have trouble with his opinion of you.
