Firstly:

It is not permissible to wear amulets, even if they contain words from the Holy Qur'an, because of the general meaning of the words of the Prophet (blessings and peace of Allah be upon him): "Whoever wears an amulet, may Allaah never fulfil his wish and whoever wears a seashell, may Allaah never protect him from what he fears." Narrated by Ahmad (17440); classed as hasan by Shu'ayb al-Arna'oot in Tahqeeq al-Musnad.

And he (blessings and peace of Allah be upon him) said: "Whoever wears an amulet has committed shirk (i.e., associated something with Allah)."

Narrated by Ahmad and al-Haakim; classed as saheeh by al-Albaani in Saheeh al-Jaami', no. 6394.

If amulets contain verses from the Qur'an, there is a scholarly difference of opinion concerning that, but the most correct opinion is that they are not allowed, because of the general meaning of the evidence, and so as to block the means that may lead to shirk, and because that usually involves treating the verses with disrespect, because people usually sleep wearing the amulet, or enter the washroom with it, and so on.

It says in Fataawa al-Lajnah ad-Daa'imah (1/212): The scholars are unanimously agreed that it is haraam to wear amulets if they contain anything other than words from the Qur'an, but they differed if they do contain words from the Qur'an. Some said that it is permissible to wear them and others said that it is not permissible. The view that it is not permitted is more likely to be correct, because of the general meaning of the hadeeths and so as to block the means that may lead to shirk. End quote.

See: Question no. 10543 and 20349

Secondly:

If the amulets that are written by this imam contain any elements of shirk, such as asking of anyone other than Allah, may He be exalted, among the jinn or angels or righteous people, or this imam is one of those who practice this kind of shirk, or he claims to have knowledge of the unseen, it is not permissible to pray behind him. But if it is free of shirk and he is one of the people of Tawheed (i.e., he affirms the Oneness of Allah), then prayers offered behind him are valid. It is essential to advise him in either case, because amulets of all types are haraam, as stated above.

The Standing Committee for Issuing Fatwas were asked about praying behind someone who writes amulets for people, and they replied: It is permissible to pray behind one who writes amulets from the Qur'an and du'aa's prescribed in Islam, but he should not write them because it is not permissible to wear them. But if the amulets contain any elements of shirk, one should not pray behind the one who writes that and it is essential to explain to him that this is shirk. The one who should advise him is the one who knows that he does that.

End quote from Fataawa al-Lajnah ad-Daa'imah, 3/65

Shaykh Saalih al-Fawzaan (may Allah preserve him) was asked about praying behind someone who writes amulets, and he replied:

With regard to what you have mentioned about writing amulets, amulets are subject to further discussion. If these amulets contain phrases of shirk, supplication to anyone other than Allah, may He be glorified and exalted, and unknown names, it is not permissible to write them or use them, according to scholarly consensus, because they are shirk. One should not pray behind this man (who writes such amulets). But if these amulets contain words from the Holy Qur'an, permissible du'aa's and du'aa's narrated in reports (from the Prophet (blessings and peace of Allah be upon him)), then this is a matter concerning which there was a difference of opinion among the scholars. Some of them regarded it as permissible and others said that it is not allowed, and not allowing it is more on the safe side, because opening the door to writing them and wearing them is a means that may lead to haraam types of amulets, and because writing the Holy Qur'an in the form of amulets exposes it to disrespectful treatment and taking it into places where it is not permissible to take it. But there is nothing wrong with praying behind the one who writes them.

To sum up: if writing amulets involves phrases of shirk, unknown names, calling upon anyone other than Allah or seeking the help of the devils, created beings or jinn, these are phrases of shirk, and the one who writes them or uses them, knowing what they contain, is a mushrik. But if they are from the Holy Qur'an, to be on the safe side it is better to avoid them and not use them.

End quote from al-Muntaqa min Fataawa al-Fawzaan

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: Is it permissible to pray behind an imam who is a fraud and a charlatan, knowing that some of them can recite Qur'an very well? Please advise us, may Allah reward you with good.

He replied:

If the imam is a fraud and claims to have knowledge of the unseen or he follows myths and does evil actions, it is not permissible to appoint him as an imam or to pray behind him, because the one who claims to have knowledge of the unseen is a kaafir – we ask Allah to keep us safe and sound. Allah, may He be glorified and exalted, says (interpretation of the meaning): "Say: None in the heavens and the earth knows the Ghaib (unseen) except Allah" [an-Naml 27:65]. The same applies to one who practices witchcraft; he comes under the same rulings as the kuffaar because Allah, may He be glorified and exalted, says (interpretation of the meaning):

"They followed what the Shayatin (devils) gave out (falsely of the magic) in the lifetime of Sulaiman (Solomon). Sulaiman did not disbelieve, but the Shayatin (devils) disbelieved, teaching men magic and such things that came down at Babylon to the two angels, Haroot and Maroot, but neither of these two (angels) taught anyone (such things) till they had said: We are only for trial, so disbelieve not (by learning this magic from us)."

[al-Baqarah 2:102].

But if he commits some sins but does not do any acts of kufr such as witchcraft or claiming to have knowledge of the unseen, then prayers offered behind him are valid, although it is better to look for someone else who is of good character and righteous, in order to protect the people's religious commitment and avoid going against those scholars who say that it is not permissible to pray behind him. Sinners should not be taken as imams, but if they are already imams, it is valid to pray behind them because the people may be being tested with them, and it may be necessary to pray behind them. But in the case of one who calls upon anything other than Allah, or who seeks the help of the dead, one should not pray behind him, because by doing these things he comes under the same heading as the kuffaar, as these are the acts of the mushrikeen whom the Prophet (blessings and peace of Allah be upon him) fought in Makkah and elsewhere. We ask Allah to set the affairs of the Muslims straight, to bless them with understanding of their religion, and to appoint over them the best of them, for He is All-Hearing, Ever Near.

End quote from Majmoo' Fataawa Ibn Baaz, 9/278

And Allah knows best.