The stance of Entente towards the new states in Central and Eastern Europe as well as towards the dissolution of Austria-Hungary (A-H) was a bit problematic.

First off, it is important to realize what actually happened during the war and describe the road to independence of these nations.

The attitude of individual countries towards the question of what to do with Austria-Hungary varied and changed throughout the war depending on the military and political situation. At the beginning of the war in 1914 only Russia and Serbia seriously contemplated any significant changes to the structure of the Empire - it was to be reformed into a tripartite monarchy, that is Austria-Hungary-Bohemia while Galicia was to be ceded to Russia and most of the Austro-Hungarian territories in the Balkans were to be given to Serbia. However this variant did not seek to dismantle the Monarchy completely.

Britain and France were less interested in reforming let alone dissolving the A-H than to decouple it from Germany and to persuade countries that were interested in A-H teritorry, namely Italy and Romania, to enter the war. That doesn't mean that there weren't proposals to do so, both the Czech and the South Slavic émigrés lobbied for indepedence^1 but France and Britain only made rather vague promises of reform and were not willing to commit to anything concrete. The main foe, unlike in the case of Serbia and Russia, was Germany, not A-H and its treatment of nationalities was from French and British perspective purely internal matter and not something of a particular importance to them.

Only after the failure of secret separate peace negotiations between A-H and the Entente in 1917 and continuous agitation by the Czech/Slovak and South Slav exiles did France and Britain start giving concrete and binding promises^2 to these exiles, however they were still mostly in terms of increased autonomy within A-H rather than dissolution, although the latter did not appear as outlandish as it did before. Mind you that both France and Britain also had to take into consideration the fact that for example Italy had been promised territories in A-H, which were also claimed by the South Slavs thus making the matter much more complicated. Even Wilson's Fourteen Points in early 1918 make no mention of A-H dissolution, only for the people of A-H to be accorded opportunity to "autonomous development".

The key development that pushed the Entente towards the idea of dissolution was the so-called Sixtus Affair and its aftermath. Essentially the secret peace negotiations between A-H and Entente were published by the French prime minister Clemenceau after Czernin, the Imperial Foreign Minister of A-H, in a speech accused the former of obstructing peace negotiations between Entente and the Central Powers. This led to A-H being discredited vis-á-vis Germany and meant a definitive end to any separate peace treaty with the Entente as well as the A-H binding itself to Germany even more. The Entente, seeing that any peace dialogue with A-H is no longer possible started backing the more radical demands of the émigrés. These proposals materialised during the so-called "Congress of Oppressed Nations" held in Rome in April 1918 among the non-german, non-hungarian émigrés of the nations of A-H. The conclusions of the congress included a demand for independence for the nations of A-H which was finally fully endorsed by the Entente.

As the war progressed towards its end, the domestic politicians of the A-H nations finally joined the émigrés in pursuing indepedence of their nations. Charles I tried one last time to save the Monarchy by issuing the so-called Manifest of Nations (Völkermanifest) which essentially was an attempt at federalization of A-H, however it was too little too late. Austria-Hungary disintegrated and the Entente subsequently recognized the newly created states.

Now to finally answer your questions: As I wrote in the previous paragraphs the Entente made concrete promises in 1917-1918 that simply could not be ignored. The priority during those years was to end the war first and the post-war order would be fully decided only after. Since A-H collapsed, there wasn't really that much of an alternative to these nations.

Moreover after the war the principal concern was to create such European order that would contain both Germany and Soviet Russia. There was a quite a bit of disagreement among the Entente on how to actually achieve that with Britain preffering the standard 19th century balance of power while the French were more inclined towards their own system of alliances with the newly created nations acting as "watchdogs" against both Germany and Russia. Both France and Britain also had other concerns than just Germany and Russia - Britain was afraid that the French system would empower France way too much and jeopardize the balance of power, meanwhile for the French the new nations meant also economic and imperialist opportunities.

As to whether they were aware of a potential ethnic conflict within these states - Yes, they were, the US and the British in particular. Both generally tended towards the creation of more homogenous entities rather than heterogenous ones, however in their efforts they were limited by the French as well as by the fact that ethnic borders were not always viable for economic or strategical reasons.

There was however also an attempt from the Entente to moderate the ethnic tensions inside the new countries by making them sign various treaties regarding minorities, for Czechoslovakia and Yugoslavia it was primarily the "Treaty between the Principal Allied and Associated Powers and Czechoslovakia" for the former and "Treaty between the Principal Allied and Associated Powers and the Kingdom of Serbs, Croats and Slovenes" for the latter within the context of negotiations in Saint-Germain-en-Laye in 1919. Czechoslovakia also had a separate treaty with Austria regarding the treatment of minorities.

---

^1 It is worth noting that there were competing concepts about what to do with Austria-Hungary between the émigrés and politicians that stayed with home. In the case of the Czechoslovaks for example, the domestic political scene was overwhelmingly against the idea of indepedence unlike the émigrés led by the future Czechoslovak president Masaryk that lobbied hard for it.

^2 These promises were mainly an attempt to turn the nations of A-H against the Empire and thus helping to end the war. It was deemed important to do so, as Russia after the October revolution withdrew from the war and thus releasing both German and Austro-Hungarian troops from the Eastern Front. Moreover in the case of Czechoslovakia, there was an active unit of Czechoslovak deserters in Russia fighting for Entente, the so-called Czechoslovak legion, which potentialy could be used against the communists.

sources:

CORNWALL, Mark, The Undermining of Austria-Hungary, 2000

RAUCHENSTEINER, Manfried, The First World War and the End of the Habsburg Monarchy, 2014

RYCHLÍK, Jan, Češi a Slováci ve 20. století, 2012

WANDYCZ, Piotr Stefan, France and her Eastern Allies, 1919-1925, 1962