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_ ... The inverted forms sound quite improbable.
4. It is rarelly done in practice. You haven't encountered it, because it is quite rare :-) The register would be _pedantic_, at least when speaking. Note also that it is rare as a _present tense_. Who would say _dors-je?_ (now)? There are only few situations where you can actually use this construction in a meaningful way. ngrams gives a peak frequency for _parlé-je_ of 0.0000016042%... (5 zeros after the decimal point) in the 19th century, and it's all downhill after that to 0.0000001012% in 1999 (that's six zeros after the decimal point). ngrams does not cite any book reference for _parlé-je_ either.
5. Yes, it is fairly indistinguishable, and we think of the _imparfait_ way before we realize it could possibly be a present with an inverted "_-je_".