But surely it doesn't make sense to expect this to be true of every
individual clause, even fully subscribing to Buchanan's view.
Individual clauses could treat X better than Y, but could be balanced
by different clauses that defend Y from X.
You could interpret Buchanan in this way, but on my reading he'd strongly resist your approach. Constitutional horse-trading between people who want X and others who want Y would seem too much like ordinary legislative bargaining for his taste. If Buchanan's veil of ignorance view is right, there's no need for horse-trading, because everyone or almost everyone reasonably expects to gain from each provision. Thacker continues:
Even more so, if the answer of who expects to gain should be "almost
everyone," I would think that an answer of "everyone except for the
king" would actually be fairly close to meeting that. Certainly under a
"veil of ignorance" most people would not expect to be king.
Yes, but lots of people might expect to be part of the "pro-king faction." And in any case, Buchanan's veil of ignorance isn't Rawls'. If you're already the king, Buchanan doesn't tell you to forget your position. He just says that constitutions will (should?) set rules under which everyone, including the king, can reasonably expect to benefit.
I think most of these clauses are what I would want if I didn't know
what station in society I'd be born into. That said, obviously the
mechanism is that locking in limits to the king power over the nobility
is the mechanism through which that is working. Since the chance of
being king is very small and the benefits to being corrupt aren't that
great compared to the burdens of a corrupt king, in a veil of
ignorance, we'd want weaker kings and strong nobility.
Again, this conflates Buchanan's veil of ignorance with that of philosophers like Rawls. Buchanan doesn't say, "Pretend you don't know your station in society, then choose rules that best protect your interests." He says, "Given everything you know, choose rules that you expect to best protect your interests, even if the rules potentially work against you." His standard example is accident liability rules. While you know a lot about yourself, you don't know whether you're going to be a perpetrator or a victim of an accident. Almost everyone else is similarly unsure about what accidents are in their future. Due to this non-hypothetical ignorance, Buchanan thinks it's feasible to reach near-unanimous agreement on accident liability rules.
In other words, Buchanan's veil of ignorance isn't a game of "let's pretend." It's an attempt to leverage our existing uncertainty into unanimous or near-unanimous agreement about something.
Christians at this time were forbidden by the church to charge interest
on loans. Which economically meant loans were hard to come by. That
important economic function, by default, fell on the Jews who were
allowed to collect interest. Quite the opposite of the sentiment of
Bryan, clause 10 seems to affirm the economic reality of the times that
Jews could lend money with an interest rate, and collect that interest.
Even if the Jews had a legally-supported money-lending monopoly, it's still inefficient to require all debt contracts to suspend interest payments to underage heirs. This is no different than a law saying that a car monopolist can only sell Ferraris. The monopolist responds by charging a higher price, and consumers willing to buy the lower-quality product for a lower monopoly price are out of luck. In any case, as long as Jewish money-lenders still competed with each other, the monopoly model wouldn't have been relevant. Clause 10 still looks like classic anti-Semitic demagoguery to me.
P.S. If you're having trouble deciphering the archaic language of the Magna Carta (I know I am), this glossary is very helpful.