Thank you very much indeed for this charming story. It's fascinating to see Hogwarts and the year of the Chamber of Secrets from the elves' perspective. I liked all the little details you gave about the way they run the castle - the laundry elf gossiping about Lockhart's robes; the clock that showed where each professor was and what they were doing; and the hierachy among the elves themselves, just to name a few things.
Caster rang very true as the responsible, chief elf. I liked his protectiveness of being the one who serves the Headmaster, and indeed his loyalty and trustworthyness with Albus' private details; Albus Dumbledore had always had a weakness for blonds, although Carster would never have admitted that to anyone, ever.
The glimpses we see of Albus and Filius' relationship were lovely. There was a sense of quiet companionship, but also of long-established trust: of course they would always be in each other's rooms together; of course they would work out the matters of the day and the academic year as a team. It's a comfort to think that Albus and Filius had one another during many of those difficult times.
Indeed, I'm a fan of several Albus/ and Filius/ slash pairings, but have never before seen them paired together. It makes perfect sense, though! I'll be hunting for more Albus/Filius in the world, now :-)