Is it permissible to combine madhhabs (schools of thought) in a single act of worship, such as prayer, whereby the worshiper follows one madhhab in part of it and another madhhab in another part, and would this act of worship be valid?

A person should follow what is indicated by the Qur’an and Sunnah, or emulate someone whose knowledge and piety they trust. They are not obliged to adhere to a specific madhhab (school of thought). There is no objection to adopting rulings for a single act of worship from different madhhabs, such as the takbirat al-intiqal (transitional takbirs) according to the Hanbalis. This does not fall under the blameworthy talfiq (piecing together/synthesis) in Islamic law. Blameworthy talfiq is taking the validity of an action from two madhhabs after it has been ruled invalid by each one individually in the same issue. However, taking the sayings of imams on multiple issues is not talfiq.