This is one of rare examples in Slovenian literature for euphonium. It is neoclassical concerto with all neccessary features: sonata form, easy-to-follow motifs, transparent orchestration and, frank, even clear tonal centre G major. I was a bit inspired by Hindemith's "Gebrauchmusik" since there is not enough concertos for euphonium to venture into experimental world of art (which is, frequently, terrible for ears and mind as well).
Pretty lovely although at around 1:15 the progression is way too banal (A-b-G-D). I prefer other moments where harmony is more open although firmly in tonality. Nice, memorable motivic features also help the musical development.
I don't own the movie and don't feel like watching the whole thing, so I just watched a quick bit of that scene on YouTube. From the comments there, I want to make sure by asking a question: Is the music Mazurka from Life of a Czar? It sounds lively, but to my ears, it has a different style from Chopin. Sorry about that.
My favorite is the first movement, which like the others comes across as a more deterministic rubato than rubato. It's relaxed at these tempos and ambiguous, which is very charming I think. But because you have imposed this extra formal procedure on it, I have to re-adjust my reaction to it. Which is pleasant and which I am very happy to do.
I would love to hear a wicked up tempo piece from you using these same techniques.