What is the ruling on the fasting of one who combines, in every detail of the issues of fasting, the opinions of different madhhabs? Is fasting considered a single issue, or is each detail within it an independent issue? And does acting upon a combination of madhhabs (talfeeq) in acts of worship invalidate the act?

The blameworthy talfiq (piecing together rulings from different schools) is that which occurs in a single issue. An example of this is performing ablution (wudu') in a way that would be invalid according to one of two schools but valid according to the other. This is like someone who touches a woman and experiences bleeding. Their ablution would be invalid according to the Shafi'i school due to the touch, and according to the Hanafi school due to the bleeding. The validity of their prayer would then be pieced together from both schools simultaneously, and such a pieced-together ruling is invalid by consensus.

As for talfiq in multiple issues, or adopting the opinion of a school in a detail that does not contradict the first school, this is not blameworthy talfiq. An example of this is someone who performs a complete ablution according to the Shafi'i school, then touches a non-mahram woman. They are permitted to pray with that ablution, considering that it is not invalidated according to the Hanafi school.

Regarding the identification of the questioner's madhhab (school of thought) before answering, this is not necessary, and the questioner may not even be following a specific madhhab.