Get advice from (non insurance) company lawyer before you give evidence against any oprocess with a former employer.
Be clear that you do not breach any duty, implied or express, in trhe contract of employment.
Be clear you dfo not leave yourself oipen to employer coming after you - for any reaosn - while the insyurance company clears off.
Good luck. I hope you succeed. I hope you give evidence. Too many companies gt away with bad practice. Only former employees are truly in position to expose same.
To be paid for the evidence? I would say 'No.' They could treat you as some type of consultant/advisor in the matter, depending on the technicality, complexity etc. You coukld reasonably expect to be compensated for days away from work, or for travel and subsistence. Be careful for yourself on everything else.