Yes, more or less, but I would not include the particle は in identifying a phrase like that. The nominalizing こと I would not include here, because including it would instead mean "devoting (oneself) single-mindedly" which is not quite the same thing.
Also oneself is a strange thing to use here when you know who the subject is in the context of the sentence.
You misunderstand what Chris-san wrote. He said that "to devote oneself single-mindedly" is a verb phrase, so the underlined part should be just 一途に献身する. If you include こと there, it works as a noun phrase like "devoting (oneself) single-mindedly", not a verb phrase. Also, は shouldn't be included in a phrase like that, either.