I am not much aware that inversion is common in colloquial speech, i d say no, but there are a lot of differences between colloquial southamerican and european castillian (to say spanish, at least here, has a lot of political implications - and all of them negative). On the other side, for emphasising the phrase is quite acceptable (it is more poetic, discursive, etc than colloquial).
The use of verb "ir" which in 3 person present is el/ella va , is a kind of modism used to indicate that something is in the process to get its solution/result, etc.....if you say: el verano està llegando" it is a statement that reflects a fact. "el verano va llegando" shows more a feeling/sensation of the speaker. It seems is coming, but you dont state it as a fact. just as your perception of the moment (because it is warm, you see the sparks, etc....)