Is my testimony considered false witness if I attest to the judge that I witnessed an incident that I did not, even if I rely on the husband's own admission and the testimony of trustworthy individuals, and what is the expiation for this testimony if it was false witness, after more than ten years have passed?

The testimony must be based on observation and certain knowledge, as Ibn Qudamah said, due to Allah's saying: "but only he who testifies to the truth and he knows" and "And do not pursue that of which you have no knowledge." So, if it is certainly proven to you that what you mentioned occurred, then your testimony is not a false testimony. However, you would be sinful for informing the judge that you witnessed the incident, and this is a lie whose expiation is sincere repentance.