Please help me understand this document so I can trace my grandfathers past

https://www.familysearch.org/ark:/61903/3:1:S3HY-DB17-Y5K?view=index&action=view&cc=1609792

This link is a pasenger list dating pack to 1927, and after days of frustrating research I have a hunch that two passengers on line# 12 and 13 could be my grandfather and his mother...Costello is the sir name.

Now, my understanding of these records is that the ship departed Vancouver and was destined for Tahiti...snatching up passengers from various other ports along the way? And it appears the Costellos boarded the ship in Suva Fiji. Forgive me if im misinterpreting the information on this document, but thats the way I read it. Please correct me if im wrong.

Im not totally certain the passengers are my grandfather and his mother ofcourse, but the Costello name is not a common name here in New Zealand, and the only records i can find of him are death records and police records dating back to when he was in his early 20s. This Passenger list is the only other record i can find which matches the sir name with the exact birth year, only issue is that theres no first names on the list to join the dots.

Is there any way possible I could track down in more detail who exactly who boarded that ship?

Any help is greatly appreciated.