There's no single correct answer. I do not know for certain exactly how this would be answered for a provincial court employee in Canada either.
I am quite confident that whichever scheme you choose above, your mother will not have any issues. Just explain and provide title. IRCC is mostly only concerned about 'command' positions, and only barely concerned about roles that are not military/paramilitary.
Eg employed as an assistant/clerk, who is considered three ranks below a local district judge (reported to the court stenographer.)
Do not include the rank of 'peon' though, just leave that out.