When is a jurisprudential issue considered one of difference, such that it is permissible for a Muslim to adopt one of the opinions therein, and what is the basis upon which a choice is made between these opinions?

Accepting statements and giving them credence depends on their accordance or divergence with the evidence and scholarly consensus (Ijma'). The issues mentioned by the questioner, such as musical instruments (ma'azif), congratulating disbelievers, and shaking hands with a non-mahram woman, are reported to have an Ijma' (scholarly consensus) on their prohibition. This differs from the issue of giving monetary value for Zakat al-Fitr, where each opinion has its basis and evidence. It is imperative to differentiate between a permissible and valid difference of opinion (khilaf sa'igh) that does not contradict a text or Ijma', and an impermissible difference of opinion (khilaf ghayr sa'igh) that relies on personal whims, mere opinion, or a far-fetched interpretation of evidence.