So I wanted to change my ship after taking the mission, since the mission suggested I should have a ship with some cargo space, and after I unloaded the stuff I was given for purchasing the 1M ticket (deneb rum and some other stuff I believe) and selected another ship, the mission suddenly completed. Happened a few mins ago on my Itani alt.

Did you happen to leave the sector after arriving at the embarkation point? There is the following warning given in the mission text:

While waiting to board do not leave this area as your ticket will be voided.

Or, is there any way the mission may have been aborted by accident? The completion status is showing up as "Aborted" which is very odd (I think you can verify this under your Mission Logs).

Nope, I don't think I went anywhere. In my mission log the title of the item is "Safari Cruise (Completed)".