Is it forbidden to combine different opinions (talfiq) in following what is true and supported by stronger evidence, rather than whims? And are my previous prayers valid regarding the issue of nullifiers of ablution, knowing that I only recently became aware of Ibn Hazm's opinion?

In Arabic, "talfīq" (piecing together/fabrication) linguistically means the combining of two things. In jurisprudential terminology, it refers to performing a single action by combining several schools of thought, such that the action would not be considered valid according to any one school of thought individually. Scholars have differed regarding the ruling on talfīq. Some have held that it is impermissible, while others have allowed it, and this latter view is the more preponderant, especially for common people for whom anything else would be difficult, or if it occurs by اتفاقًا (coincidence) and not with the intention of pursuing dispensations.

As for adopting the opinions of imams in multiple issues, this is not talfīq; rather, it is moving between schools of thought, and it is permissible unless the motivation is to follow personal desires. If a person follows a school of thought on an issue and then changes it for the same issue, it is permissible if it is due to the emergence of truth, and prohibited if it is following personal desires.

The common person should refer to trustworthy scholars. If a scholar he trusts gives him a fatwa, he must follow that fatwa. It is not permissible for him to pursue blameworthy dispensations unless another opinion becomes clear to him as more preponderant, or if the second scholar is more knowledgeable and pious.

As for the discharges that exit from a woman, they are pure but invalidate ablution according to the majority of scholars. If they are continuous, she should perform ablution for each prayer after its time has entered, and she is permitted to combine prayers when there is hardship. Touching maharim (unmarriageable kin) does not invalidate ablution. Your previous prayers that you performed, following a school of thought that does not consider discharges to invalidate ablution, are valid.