Is imputation a legal fiction?

The stock objection to imputation (or vicarious atonement/penal substitution generally) is that you can't detach the merit/demerit of a deed from the agent who performed it and reattach it to a second party.

There's no knockdown argument for that. It's just the intuitive sense that imputation artificially separates what's inseparable.

One counterarguement I've used in the past goes like this: A defining element of friendship is favoritism. We do things for friends we don't do for strangers. Any particular favor is gratuitous; however, that's grounded having earned someone's friendship, and vice versa.

But there's an extension to that principle:

Bud is Bubba's friend

Bubba is Buster's friend

Buster isn't Bud's friend

(They don't dislike each other, but they don't happen to be friends.)

Bud asks Bubba to do a favor for Buster. Bubba complies, but he's really doing the favor for Bud. He is treating Buster as if Buster is Bud; he is treating Buster as if Buster is his friend, in virtue of his friendship (and their mutual friendship) with Bud. So there's a transferable dynamic. 

Now for a different kind of argument. In criminal law and theological ethics, it's typical to distinguish intent from the objective character of the deed. Not only are they distinct, but separable.

For instance, if a 4-year-old shoots his 5-year-old brother to death, we don't charge the 4-year-old with murder. Although the deed is objectively wrong (i.e. his brother's death is an evil), and even though action may have been deliberate rather than accidental, we make allowance for the fact that at that age he's incapable of forming criminal intent. Even if he was mad at his brother and wanted him dead, he didn't appreciate the significance of the act. He didn't want his brother to stay dead. He didn't expect his brother to stay dead. He doesn't think long-term. At that age he can't.

Consider to other examples: 

Jim and John are bunkmates on a military base. Jim works in military intelligence. Jim is a patriot. Jim and John are best friends.

Unbeknownst to Jim, John is actually a spy. Because he thinks John is trustworthy, Jim sometimes shares things is casual confidence regarding military secrets. Indeed, because John is bit of a math genius, Jim sometimes asks John to help him out on decryption/encryption.

As a result, Jim unwittingly compromises national security. His action is objectively wrong, but because he did not intend to compromise national security, the absence of malicious intent is either exculpatory or a mitigating factor . He had no reason to suspect John's bona fides. That's an extenuating circumstance.

In addition, friendship is praiseworthy. Although John betrayed his trust, it's the kind of betrayal that can only take place in the context of friendship (or apparent friendship), and friendship is virtuous. 

A final example: Drake is driving on a country road during a rainstorm. He spots a pedestrian who's getting drenched. He offers him a ride into town. Unbeknownst to Drake, the pedestrian is a serial killer, on the run from the law. 

A squad car passes them and continues on its way. The policeman is on the lookout for a man matching the description of the suspect. But because Drake gave the suspct a ride, the policeman misses him. Had Drake driven past him, the serial killer would have been apprehended by the policeman.

As a result, Drake unwittingly facilitates serial murder. Thanks to Drake, the sociopath eludes capture and continues his killing spree.

Drake's action is objectively wrong. Objectively blameworthy. But because he intended no wrong, the absence of malicious intent is exculpatory. 

Moreover, it was admirable for him to pick up a stranger in the rain. 

Technically, we might say agents are blameworthy rather than their actions, unless we view the action as an extension of the agent.

A final example is an organ donor. In general, organ donation is morally commendable. However, donors usually have no control over who the organ(s) go to. That liver might go to a patient who will commit murder a decade later. 

These examples illustrate the principle that the elements of moral action, the elements which make an action moral or immoral, are detachable. The intent may be inculpable or praiseworthy even though the deed is objectively blameworthy or culpable. 

That's not necessarily the same as transferable, but it's hard to claim that what's detachable can't be transferable. 

This is a less direct parallel than the friendship/favoritism example. It about a more general principle.