You know, it occurs to me that this debate hinges on the fact that a lot of people just dont get the importance Lewis places on the medieval idea of an anointed king in the Narnia stories.
In Peter`s case its particularly important as he is THE HIGH KING appointed by Aslan himself to rule even over all the other Narnian kings.
This means that, in effect the ONLY person to which he has to justify himself to is Aslan, nobody else has the right to question his judgement or be apologised to!
Now, the High King aspect is crucial if you want to figure out what is going on throughout the film.
As to why he expects to return to Narnia, well remember "Once a King or Queen in Narnia, ALWAYS a King or Queen".
He is bing a bit childish with his "it`s been a year" comment, he should expect it to take a lifetime if neccessary but he has every reason to assume that he is in a `Once and Future King` situation and will, at some point be called back.
Once back in Narnia he has every reason to assume that his job is to take up the kingship once more as that is what he is SUPPOSED to do!
He actually starts outs rather well, leading the others in the RIGHT direction, despite some questioning of his judgement from some quarters and its only when they reach the gorge that things begin to go wrong.
Lucy, of course sees Aslan who is showing them the way to cross but He dosn`t, undermining his confidence, as a result he `goes with the majority` and follows Susan and Trumpkin`s idea of heading downriver straight in to the Telmarine army!
Now, this one of the key areas where the book and film diverge.
In the following campfire scene in the book Lucy finds Aslan then helps the others see him, this means that by the time the Pevensies join forces with Caspian they all have a clear idea what they are supposed to be doing.
In the film Lucy does meet Aslan but it turns out to be a dream (though a prophetic one). This means Peter still has some self-confidence issues which result in him making some serious mistakes in the mid point of the film.
He actually falls in to the same trap that the Narnians have, in assuming that its Peter and his ability as a general that they need to save Narnia and not as is actually the case, for the broken link with Aslan to be renewed.
Something that only Lucy really understands at that point.
He also has to deal with a Prince Caspian that appears to think that HE should be in command, that the Pevensies are just there to put him on the Telmarine throne and help him avenge his father!
After the castle raid and the near return of the witch however, he begins to see his mistake, probably helped by Lucy.
So by that point the story is more or less back on track with Peter willing to risk his life in a duel with Miraz just to by time for Lucy to find Aslan.
(And I note that the film adds a point where Susan ALSO does this!)
By the end of the film, Peter has finally met Aslan (who probably had more than a few things to say to him!) and understands that he is no longer needed, which is why he passes his sword to Caspian.
From that point onwards it will be Caspian and his heirs who will rule in Narnia, in the name of Aslan and High King Peter and NOT because Caspians father happened to be a Telmarine King.