This doesn't even begin to cover things, even for English. First of all, "catch fire" is at least partly understandable from its constituent parts - "catch" has a great variety of related meanings, and they all have to do with something taking hold of something else; I'm sure any English speaker who has encountered both words would intuit the meaning of "catch fire" without any problem, especially if they also encountered "to catch a cold". Of course, the meaning
is slightly different, and it is quite invariant.
Your analysis of the phonetic atomicity is also unsatisfactory. First, the article can very well be pronounced independently - I can say "English has a single indefinite article, with two forms: 'a' or 'an'". Secondly, the 'a' form can be pronounced in two different ways, depending on how you want to highlight it within a sentence: "he ate a piece" could use the schwa, or the "long a" if you want to highlight the article itself "he ate [ay] piece, not your piece". So the article's pronunciation can change independently of the word it is applying to. Finally, in at least some English accents, many words can be pronounced differently in certain sequences than others - for example, in modern Southern English, an "r" sound is introduced almost always in speech when a word that ends in a vowel sound is followed by another word that starts with a vowel sound, e.g. "I saw-R-it". By your description, neither "saw" nor "it" are individual word phonologically, since you don't know how they will be pronounced unless we know the following word.
Overall, the atomicity of a linguistic construct is highly debatable, even in a particular context.