I understand that an epenthetic л should always appear between any of the five labial consonants б, п, м, ф and в and the sound /j/ (except in front of the sound /ja/), so that, for example, in the verb любить (to love), the epenthetic л appears in the first person singular (люблю́), but not the third person plural (лю́бят – because of the sound я).
Given this, my question is threefold:
Why does the epenthetic л not apply in front of the sound /ja/, but does appear in front of any other sound starting with /j/. What is the phonological, orthographical or historical reason for the exclusion of the /ja/ sound ?
Irrespective of 1), why is there no epenthetic л in the conjugation of the verb звать after the в and before the ё seeing that this letter starts with the sound /j/? (specifically, why is there no л after the в in зовёшь, зовёт, зовём and зовёте) ?
Irrespective of 1) and/or 2), why is there no л in the conjugation of the verb вызвать (to call, to send – вы́зовешь, вы́зовет, вы́зовем and вы́зовете) after the в and in front of the sound /je/, whereas the same sound triggers the epenthetic л in verbs such as трепать (to wag) thoughout its entire conjugation (треплю́, тре́плешь, тре́плет, тре́плем, тре́плете, тре́плют), including in its imperative forms with the sound /i/, which should not per se trigger the epenthetic л (трепли́, трепли́те) ?