I also think that there are other ways he could have gotten to the school, but it works out that Dumbledore would want Lupin to be there because of the dementors, especially being the DADA teacher. Dumbledore would want someone there to protect Harry and the other students.

Not having someone on the train in OOTP is a good point, but i dont think the death eaters would have attacked the train on the way there, and then the way home they might have had someone there against the dementors but i think the main focus on the way home was how Harry was feeling about Sirius