While in general I agree with your point of view, I beg to difer, David, about Tchaikovsky and Rachmaninoff. I would say they were also "nationalistic", albeit in a more moderate manner. You could not by any strech of the imagination listen to the works of either and say, "This is a German composer. You would immediately say, Russian!" This goes also for Stravinsky (at least the early period). You mention the Great Gate of Kiev with the bells: Rachmaninoff was fascinated by these very bells that you say fascinate Russians! Both, by the way, contributed music to the Russian Orthodox Church, which might also say something. The same goes for Bortkiewicz.
The difference is their approach: while the "5" believed that symphonies, as well as any form of musical learning, were the the works of a West to be despised, Tchaikovsky and Rachmaninoff had a much wider outlook. Even such a Western-looking composer as Rubinstein has his "Russian" works, like the Russian Capriccio and an opera or two on Russian subjects. Rimsky-Korsakov is an interesting example, as he began as his friends of the "5", but eventually realised that lack of study did not produce good music, national or not, and "sold his soul", at it were.
I believe that what happens with Cui, Medtner and Schnittke (a modern composer, of course) is that they were not ethnic Russians, but the children of immigrants. You yourself have mentioned Medtner's "Germanness". The only one who seems to sound really Russian while being the child of immigrants is Glier (or Glière, as the composer liked to be called). I say this not because I believe it is in the blood, but because the child of immigrants often lives in a segregate society (there was, for example, a German Republic of the Volga, until dissolved by Stalin). This certainly was the case of Schnittke.
You mention Chopin and his mazurkas. This dance, as you have possibly noticed, is widespread over all the countries where there is a Slav population, which explains why Dvorak wrote some, as well as Tchaikovsky. The same goes for the dumka (plural dumki) and we have dumkas written by Dvorak, Tchaikovsky and even Chopin!
Borodin, with his Polovitsian Dances, as well as Rimsky-Korsakov in Sheherazade and Antar, are indulging in what the Russians call not "Nationalism", but "Orientalism", that is, drawing on the music of minorities within the empire or even from Arabia itself.
Liszt was also the chid of immigrants (original spelling List) and could not speak a word of Hungarian. He confused Hungarian with Gypsy (called in Hungarian Verbunkos and was the same error Brahms was to commit with such disatrous consequences to his pocket) and his Hungarian Rhapsodies should by rights be called Gypsy Rhapsodies. Enescu also fell into this trap, where, in his 1st Romanian Rhapsody, he quotes "The Lark", a very famous Gypsy violin piece which is in the repertoire of Sandor Lakatos and his Gypsy Band, for example. Bartok has the merit of having discovered the real Hungarian music, a music that has little to do with Gypsy orchestras and virtuoso violin passages.
By the way, I came across yesterday in a shop the complete piano music of Ljadov.
"Please do not shoot the pianist
He is doing his best."
Oscar Wilde: Impressions of America: Leadville