Here are some more comments:

1. I couldn't find a name for that phenomenon, besides _euphonie_.
2. I think the only reason for this is _euphonie_, to avoid a difficult to pronounce combination of consonants. 
3. Grammatically you could form _achetè-je_, _pesè-je_, but those are IMHO never used, and instead we fall back on _est-ce que j'achète_, _est-ce que je pèse_ ... but ...
4. ... it is rarely done in practice, at least when speaking. The register would be _pedantic_, certainly when speaking. Also you cannot always use this construction in a meaningful way. Who would say _dors-je?_ (now)? Also, if the inverted form is in an interrogative sentence, we would use _est-ce que_ much more readily, certainly in conversation, and avoid this inversion. 
5. I would say it is fairly indistinguishable, and we think of the _imparfait_ way before we realize it could possibly be a present with an inverted "_-je_".
6. For the cases where the vowel used is _é_, although it's written with an _é_, it's definitely not pronounced like an _é_.
7. We don't quite learn to form those expressions systematically in school. We encounter a few at some point probably in literature, but I would say that is about it. We can recognize them, but we might be hard pressed to make one. 
8. Some of these expressions are tied to a subjunctive (without the usual _que_ we associate to the subjunctive?):

 > Puissé-je en finir avec cette histoire !

 > Je tiendrai jusqu'au bout, dussé-je en mourir.

 > Eussé-je ...

 So, I have a follow-up hypothesis that these expressions (or at least those tied to a subjunctive) could function more and more these days as ready-made formulas, in limited number, that we still see sometimes in print, but wouldn't use normally when talking, or even at all.