Praise be to Allaah.

If a person who is leading the prayer or praying behind an imam makes mistakes in the recitation of al-Faatihah in such a way that he changes the meaning of the verses, then his prayer is invalid, because al-Faatihah is one of the pillars or essential parts of the prayer, so he must recite it correctly and learn to recite al-Faatihah properly, unless he is unable to do so, after trying hard, for Allaah does not burden any person beyond his scope. But if he is an imam, no one should pray behind him except those who are like him or less able than him to recite al-Faatihah properly. 

Al-Nawawi (may Allaah have mercy on him) said: It is makrooh for one who makes grammatical mistakes when reciting the Qur’aan to lead the prayer, but it is subject to further discussion. If he makes mistakes that do not change the meaning, such as using incorrect vowels in the phrase al-hamdu Lillaah, then his prayer is valid as is the prayer of those who follow him. But if it does change the meaning, such as saying an’amtu (first person) instead of an’amta (second person) ‘alayhim, then his prayer is invalid. If he is able to learn how to pronounce it properly, then he must do so, but if he is unable to do so and there is not enough time, he should pray and make it up later on, but it is not permissible to follow him (in prayer). 

But if he is not able to learn or he has not had enough time to learn, if that is in al-Faatihah then the prayer of others like him that is offered behind him is valid, but if people who can pronounce it properly pray behind him, it is like the prayer of a literate person offered behind an illiterate person (i.e., the prayer is not valid). If it is in something other than al-Faatihah then his prayer is valid, as is the prayer of those who pray behind him. End quote from Rawdat al-Taalibeen (1/350).

Ibn Qudaamah (may Allaah have mercy on him) said: “If an illiterate man leads another illiterate man and a literate man in prayer, the literate man has to repeat his prayer” – the illiterate man is one who cannot recite al-Faatihah or part of it properly, or he mispronounces one letter of it, even if he recites the rest of it well. It is not permissible for one who can recite it well to be led by him in prayer, but it is permissible for others like him to be led by him. 

Then he said: Whoever omits one of the letters of al-Faatihah because he is unable to pronounce it, or he substitutes another letter for it, such as the person with a speech defect who substitutes ghayn for ra’, or who contracts one letter into another, or who recites with grammatical mistakes that alter the meaning, like the one who uses the wrong vowel in the word iyaaka and says iyyaaki instead, or says an’amtu instead of an’amta, and cannot correct it – such a one is like an illiterate person, and it is not permissible for a literate person to be led by him in prayer, but it is permissible for any of them to lead another like him in prayer, because they are both illiterate, so it is permissible for one of them to be led by the other, like two who do not do a thing well. But if he is able to correct any of that but he does not do so, then his prayer is not valid and neither is the prayer of one who is led by him. 

He also said: It is makrooh for one who makes grammatical mistakes when reciting but does not alter the meaning to lead the prayer. This was stated by Ahmad, but it is valid for him to lead one who does not make mistakes when reciting, because he has fulfilled the obligation of reciting. If he alters the meaning in something other than al-Faatihah that does not mean that his prayer is not valid and that he should not lead others in prayer, unless he does it deliberately, in which case the prayer of both is invalidated. 

If his mistakes do not change the meaning of the verses, then it is permissible to pray behind him, but it is obligatory to teach him how to recite properly. If his mistake is in something other than al-Faatihah, it detracts from his prayer but does not invalidate it, but it is undoubtedly better to pray behind one who recites properly, and it is not permissible for those in authority to appoint such ignorant people to lead the people in prayer otherwise they will have a share of the sin with them. See: al-Mughni (3/29-32) 

The scholars of the Standing Committee said:  

… But if he makes mistakes, if his error is grammatical but does not alter the meaning, then it is better to pray behind someone who does not make grammatical mistakes if that is possible. But if his grammatical mistakes in recitation al-Faatihah alter the meaning, then prayer offered behind him is invalid. That is because of his mistakes, such as saying iyaaki na’budu instead of iyaaka na’budu or saying an’amtu ‘alayhim instead of an’amta ‘alayhim etc. If he makes mistakes because he has not memorized it properly, then someone else who has memorized it better is more entitled to lead the prayer than him. 

Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Ifta’ (2/527) 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked: 

There is an imam who makes grammatical mistakes when he recites the Qur’aan, and sometimes he adds to or takes away letters in the Qur’aanic verses. What is the ruling on praying behind him? 

He replied: 

If his mistakes do not alter the meaning, there is nothing wrong with praying behind him, such as if he says Rabba or Rabbu instead of Rabbi in the phrase al-hamdu Lillaahi Rabbi l-‘aalameen; or if he says al-Rahmaanu instead of al-Rahmaani etc. But if he his mistakes changes the meaning, then people should not pray behind him if he does not benefit from prompting, such as if he says  iyaaki na’budu instead of iyaaka na’budu or saying an’amtu ‘alayhim instead of an’amta ‘alayhim. But if he accepts being taught and corrects his recitation when corrected, then it is valid to pray behind him. What is prescribed in all cases is for the Muslim to teach his brother both in prayer and otherwise, because the Muslim is the brother of his fellow-Muslim and he guides him if he makes a mistake and teaches him if he is ignorant and corrects him if he makes a mistake when reciting the Qur’aan. 

Majmoo’ Fataawa Ibn Baaz (12/98,99) 

Secondly: 

With regard to repeating Ya Lateef one hundred times, undoubtedly this is a bid’ah (innovation) if a Muslim says it by itself, because it is a phrase that makes no sense – it is a call to Allaah but what comes after it? Is he asking his Lord for anything? Does he want to praise Him after that? That is not so. And if it is recited in unison that is another bid’ah. 

See the comments of the scholars concerning that in the answer to question no. 22457 and 26867 

And Allaah knows best.