X, along with Y, "were" or "was"?

I was interested in the following sentence which appeared in a brief Tom Daschle's biography in The Washington Post, Politics, (WHO RUNS GOV).

She, along with Mark Childress, were set to be Daschle's deputies at the White House before he stepped down.

Can someone clarify if the fragment "She, along with Mark Childress, were set to" is ungrammatical, as I think it is?

I would reword were with was because I think that the subject of the sentence - obviously, in grammatical terms - is "She", not "She" and "Mark Childress", but I'm not sure on this correction.

More precisely, I'm not so sure that "She, along with Mark Childress" is equivalent to She and Mark Childress with regard to the effects on the grammatical numbers.

You are correct. The subject of the sentence is "she", and thus the verb should be singular. It is true that, in a sense, the sentence is logically equivalent to "she and Mark ...", but "logically equivalent" and "grammatically equivalent" are two completely different things. The sentence is talking about "she" (whoever "she" is), and mentioning as a side note that Mark was also involved. It is not talking about "she and Mark".

From The American Heritage Guide to Contemporary Usage and Style:

When a subject is followed by a conjoining prepositional phrase such as in addition to, as well as, or along with, the verb should be singular:

Jesse as well as Luke likes jazz.

The old school along with the playground is up for sale.

This is also what I learned. Most of the time, it's the syntactic number of the subject, not the underlying meaning, that dictates the verb agreement.

Another notable usage guide, Practical English Usage, 3rd ed, also says the same.