Am I obliged to offer expiation for an oath or a vow I made to myself to abandon forbidden acts and not to disbelieve, due to: firstly, being forced to attend a mixed-gender wedding; secondly, unintentionally committing "minor" forbidden acts; and thirdly, the thoughts about Allah the Almighty that crossed my mind, which I did not affirm?

It is obligatory for a Muslim to avoid all sins, both apparent and hidden. Whoever makes a covenant with Allah to abandon a sin, then commits it, must, in addition to repenting, offer an expiation for breaking an oath (kaffarat yamin). However, if the covenant was only with oneself, then no expiation is required, but it is obligatory to fulfill it, as refraining from forbidden acts is mandatory. If you suffer from obsessive thoughts (waswas) and doubt whether you made a covenant with Allah or swore an oath, then no expiation is required of you; this is because the fundamental principle is freedom from liability, and certainty is not removed by doubt. Furthermore, a person is not held accountable for thoughts that cross their mind unless they act upon them or speak them.