Casting an opponent's commander

Question: In the Commander game format, is it possible to construct a situation where you can cast an opponent's commander from the command zone?

Of course the answer is "yes" (and the image below spoils the answer), but the interesting rules question is why it's possible.

One way to achieve that is to make the opponent's commander go to the top of that opponent's library (for example with Excommunicate), and then attack with an unblocked Daxos of Meletis. After that, even if your opponent decides to put the commander in the command zone instead of exiling it, you can nevertheless cast it.

This is a little quirk of the rules of the game (and is in fact something that WotC fully acknowledges as a valid thing to do. In fact, a recent rules change clarifies the situation, making it even clearer.)

The key reason why this is possible is because the effect uses the wording "that card", rather than for example "the exiled card". (For example the same would not work with something like Oblivion Ring, which says "when Oblivion Ring leaves the battlefield, return the exiled card to the battlefield under its owner's control." Because it uses that wording, if the card is not in exile when the ability resolves, it won't do anything. This is delineated in the rules of the game.)

Because Daxos uses the wording "that card", if the exiling is replaced with the card going to another public zone, the effect will keep track of it (until it changes zones to somewhere else, after which it loses track of it because it's now a different object.) Note that it won't be able to track it if the exiling is replaced with going to a hidden zone.

So the next logical question is: What about the commander mana tax? Do you have to pay 2 extra mana for each time it has been cast from the command zone?

You don't pay the owner's mana tax. Instead, you have your own mana tax imposed for that particular commander (and which is independent from the mana tax of your own commander.) In other words, the first time you do this, there is no tax. However, if you manage to repeat the same feat with the same commander during the same game, you'll now have to pay 2 extra mana to cast it (and so on.)