As you can see, it should be quite obvious, why the full list of merit sources is not publicly available anywhere, and why only Staff can see it.

I wouldn't be so sure. If someone can receive similar amount of merits, that he is rewarded with, then by looking at received and sent merits, you won't figure it out, that such person is a merit source. You could catch someone, if he will ever send more than half of what was received, but this is the only way to detect it with 100% certainty.

Also, there are users with thousands of unspent merits. But very often, there is just not enough posts, which are worth meriting (or they are made by Legendaries, who have more than enough merits). And many people don't want to spend all merits they have, because then, merits are worth less than usual, if they are distributed too generously. Also, if you go to zero, then when you will notice some really good post, you wouldn't have enough merits to use them. And because you cannot "timelock" merits, you cannot spend them now, and see them sent in the future, when you will receive new merits, or when they will be allocated, if you are a source.

By the way: maybe that idea with timelocking merits is worth considering. Of course it should be protected from flooding, and going into "-1000000 merits, waiting to be received and sent", but I can see a value in going for example to "-10", and having a private history of pending "transactions", which would be visible only to the account owner. I guess it could be a good test, to see, if someone deserves becoming a source, without explicitly granting that permissions; maybe even being a source should work in that way.