Is it permissible to pray behind an imam who does not obligate the recitation of Al-Fatiha, as he follows the Hanafi school of thought, while the worshipper considers it obligatory, being a follower of the Shafi'i school? Is there any issue in differing from the imam regarding the number of salutations in the funeral prayer? And is it obligatory for the worshipper to follow his own madhhab or the madhhab of the imam?

It is permissible to follow a dissenter in secondary issues. According to the Shafi'is, a Shafi'i's prayer behind a Hanafi is valid, as long as the Hanafi Imam recites Al-Fatiha, because it is obligatory in his madhhab (definition). Even if there is doubt about his recitation of it, the prayer is valid according to the authenticated opinion. Similarly, a Shafi'i may pray behind a Hanbali Imam who performs only one taslima in the funeral prayer, even though the Shafi'is hold that two taslimas are recommended.