As far as the Vatican's nationhood goes, where exactly is that stated? The article I linked to in the beginning explicitly says that the Vatican was not granted nationhood by the UN, it could be wrong, but I'd like to see your source if that is the case. I realize they were granted observer status, and that implies nationhood, but that's only an implication.
Others have already addressed this from another angle, but even so, legal implication holds the weight of law, even in treaty law.
Relocation does not prevent prosecution, but it sure as hell makes it a lot less likely and more difficult to pull off.
Reassignment after an embarrassment is common. And for that matter, it's not like the church kept quiet on the forwarding address. If it wasn't important enough to seek extradition when it crossed jurisdictions, we wouldn't be having this conversation.
Churches and embassies have different rules by far. They're not the same. Not to mention there's a lot more churches...
You said office buildings, and that's what I responded to. You did not say churches. The ownership of the church buildings is held by the local parish, NOT the Holy See.
who made the decision to ignore secular law in favour of church law?
The first Pope, by Catholic tradition, Peter, in Acts 5:29, accompanied by Catholic doctrine that the church represents God's will on earth.
Peter and the other apostles replied: "We must obey God rather than men!"
I will point out, though, that nothing in scripture says a coverup is a good thing. In fact, it's demonstrated as bad when King David does it, and Jesus even says to "Give to Caesar what belongs to Caesar", which many Biblical scholars interpret as following the laws of your community so long as they don't require you to sin. This sort of "we've selected which verses benefit us today" approach is why I don't like organized religion. Community churches are what's described in scripture, and except for a few critical letters from Paul, they were independent of each other. If they stayed small, instead of pooling together in massive uncontrollable groups that insist on mindless slaves for members and forcing people to join for fear of being "the enemy" (like organized churches, multi-corporation unions, political parties, etc.), we wouldn't have these problems (or the Inquisition, or the massive industry strikes, or the last several Presidents...).