While the basic premise seems fine (narnia-verse, a group of royal heirs oppose the crowning of an evil(?) lord) i think the DM by your description sounds too attached to the events of the movie/their own script, there seems to be alot of events that are preplanned to occur and resolve in a certain way if I’m reading that correctly not accounting for the players potential influence on things
By the time Prince Caspian starts Miraz is already King and The GM decided that Miraz unofficially became King shortly before Caspian turned 3, he officially became King roughly 1 year after Caspian started his education which was roughly 1 year before Cornileus became his teacher and The GM's ruled that at the time The Film starts Caspians roughly 14 years, 7 months old, by Narnian time the fight for Caspians Throne lasts 4 months and that Aslan decided that it would be amusing to delay Caspians coronation by half a month, that way Aslans birthday present to Caspian would be him crowning Caspian
I will mention that The GM has decided that if possible, but its very likely that it won't be possible, he won't stray any further from the plot of Prince Caspian then he already has, which means that other then the attack on Miraz Castle The 5 Pcs won't be involved in any fight from the books or the films other then the final fight
This is also why he made The 5 Pcs Nobles that aren't in line for their fathers titles, the idea is that they will travel across as much of Narnia as possible and by revealing who they and their parents and, hopefully, convincing people that Caspians the rightful king they will be able to convince enough Narnian Nobles that are staying Neutral to side with Caspian then they hear of the fight and ride to Aslans How, hoping to arrive in time to turn the tides, hopefully with the numbers needed to turn the tide
Will it work?, who knows, I don't know for definite but I know The GM well enough to know that either he will have something planned or he's already got something planned that will ensure that even if they don't arrive in time to turn the tides and/or with the numbers needed to turn the tide that something will happen that ensures Caspians Victory but whatever happens that turns the tide won't be what does it in the film and book
PS in case your wondering The GM likes very strong alcohol and even though I only joined the group on the last day of April this year I've known him for almost 13 years and whenever possible I use that to get very petty revenge. See below for why
For example this time I've taken advantage of that to gain information I need to give me and the other players a huge advantage, unfair you say?, technically yes, but I had the misfortune to go to school with 2 very good friends of his, his sister, who later married 1 of the 2 very good friends, and his female cousin, who married his other very good friend and I knew them for 5 years
Because of how nasty they were to me for those 5 years I get a vindictive pleasure out of using his love of very strong alcohol to get lots of acts of petty revenge, which is the real reason why I keep deriding, insulting and belittling the guy, not nice you say?, again technically true but after what I suffered at the hands of his 2 very good friends, sister and female cousin I truly believe that I'm showing almost super human levels of both restraint and mercy