Is penal substitution possible?

A stock objection to penal substitution and vicarious atonement is that it's just not possible for one individual to assume the guilt of another individual. I've discussed this on various occasions, but now I'd like to approach it from a different angle:

i) Keep in mind that both sides have a burden of proof. The fact that critics of penal substitution/vicarious atonement raise the objection shouldn't put Christians on the defensive, as if the viewpoint of the critics is the default position. Both sides need to argue for their position. Critics are not entitled to shift the onus onto Christians. 

ii) Then there's the issue of whether this is even the kind of question we can settle a priori. In many cases, we believe something is possible because it is actual. Reality entails possibility. 

In many cases we don't attempt to justify the possibility of something a priori. Rather, we believe it's possible because we have concrete evidence that it's possible. Because we have evidence that something really happens or really exists. 

In the first instance, Christians believe in penal substitution/vicarious atonement because that's a revealed truth. Because that's what happened on the cross. That's the design of the atonement.

That's not based on intuition but experience. And that can be a legitimate source of knowledge. We rely on that for many things.

iii) What's the general principle that underlies the objection to penal substitution/vicarious atonement? In human affairs, there are many cases in which one party acts on behalf of another party, or in place of another party. 

Moreover, this may be asymmetrical. For instance, a private lacks the authority to do some things a general is entitled to do whereas a general has the authority to do whatever a private can do. So a private can't take action above his grade but a general can take action below his grade.

In that respect, a superior can take the place of a subordinate, but a subordinate can't take the place of a superior. By parity of argument, the Incarnate Son can take the place of sinners. 

iv) Consider an illustration. Suppose a gamer designs a video game with artificially intelligent characters. Most of the virtual characters are coequal. 

However, the gamer has one character who represents himself. Within the world of the game, that character has authority over all other characters, because he's the virtual counterpart of the designer. As their superior, he has the right to take the place of another or others.