In response to the opening post….no, Rav Tzadok and the Izhbitzer are not valid sources for OO changes. There are controversial concepts in this line of chassidus that are misunderstood and misapplied by many people. Most is due to a combination of (1) a failure to read very carefully and connect to related pieces elsewhere in the writings, and (2) people’s desire to find what they want to find. This has attracted a lot of attention in academic works with wildly incorrect interpretations. Shaul Magid’s book “Hasidism on the Margin” is very thorough and goes through the controversial parts (e.g. can it be Hashem’s ratzon for a person to violate halacha), and connects the different pieces in the seforim very well. After arriving at his conclusions, he notes support from the simple fact that communities following in the ways of the Izhbitzer-Radzyn hoshkafo have existed for 150 years and there have not been any deviations from halacha.
Remember that so many chassidic works were not written as seforim – they were compiled from talks, often linked to parsha. It is only from the totality of these writings that you can really understand the hoshkafic system. Even then, it often takes additional writings from disciples and successors.