Is saying, "O righteous servants of Allah, return to me such and such lost item," considered polytheism (shirk)? And must one utter the Shahada to return from this polytheism? And do all the good deeds of one who said that after it become nullified?

The saying, "O righteous servants of Allah, return to me such-and-such a lost item," is an act of seeking aid and supplication to other than Allah. The scholars are unanimous that supplicating to other than Allah for something that only Allah can do is major polytheism (shirk akbar), which is unforgivable except through repentance.

Repentance from shirk is not achieved merely by uttering the two testimonies of faith (Shahadatayn) if the person does not understand their meaning and implications. Rather, it requires believing in the invalidity of supplicating to other than Allah, its prohibition, and that it is an act of shirk. The Quran has warned that shirk nullifies deeds.

As for the ruling on the mother's deeds after the supplication and before repentance, it is a matter of scholarly disagreement. However, generally, whoever is ruled to have committed apostasy or fallen into major shirk, their deeds are nullified until they repent.

In summary: The questioner must explain to his mother the prohibition of supplicating to other than Allah, that it is major shirk, and that she must repent and understand the meaning of shirk for her repentance to be valid.

However, if the mother intends by the supplication angels or righteous jinn, this falls under seeking help from a creature in what it is capable of, and it is not considered shirk.