That is unfortunate that your mother could not get help embarking & disembarking usually they have porters to assist people with disabilities to embark & disembark with wheelchairs it is up to the port if they supply this service
Unfortunately your TA was wrong
Did you or anyone in your party read the T &C or Guest ticket contract
https://www.oceaniacruises.com/sites/default/files/2023-02/oceania-guest-ticket-contract-new.pdf
see #6