I think the most obvious answer would be that there is no doctrinal content or special meaning behind the choice of words denoting various planes of existence. In the Catholic Christianity, one also doesn’t talk about the ‘Heavenly World’, ‘Human World’, and ‘Helld World’, there are separate wolds for all supposed regions of the universe: Heaven, Hell, Purgatory, etc. No-one feels the need to unify the nomenclatory here, because it’s really not needed, everyone understands what we are talking about anyway.
So, one may assume that the word ‘loka’ means ‘place, world’, and ‘devaloka’ and ‘manussaloka’ were coined or used by the Buddha for lack of better terms. ‘Pettivisaya’ and ‘niraya’ may have been taken over from pre-existing traditions where they denoted just that: ‘Anscestral Realm’ and ‘Hell’. Anyway, your guess is as good as mine, and your theory does make sense. A similar semantic development can be observed in the Christian asceticism, where the ‘world’ is understood as opposed to the seclusion of an ascetic life at a monastery or convent. I wouldn’t say this would be exactly my intuition, as I don’t feel any strong semantic ties to the existence of a social order in words like ‘arupaloka’, but this argument is in now way conclusive. It would be interesting to see if there is any explicit connection in the Canon between the word ‘loka’ and existence of a social order in any other context apart from the one discussed.
‘Tiracchānayoni’ seems to be the least problematic of all, since, technically speaking, animals live in the same world / place / plane as humans, i.e. in the manussaloka / Jambudipa, so, to distinguish them from humans you would probably like to use expressions whose meaning amounts to ‘of animal origin’ or ‘of animal birth’ (Cf. with the use of ‘manussayoni’ in Ja506). This is also somewhat confounded by the use of ‘opapātikā yoni’ you mentioned, as it seems to mean ‘of spontaneous origin’ or ‘of spontaneous birth’. An animal can belong to both ‘tiracchānayoni’ and ‘opapātikā yoni’, i.e. to be both of animal and spontaneous origin. Curiously enough, there seems to exist the word ‘petayoni’ (even though I was unable to determine where it is used in the Canon), which, combined with the absence of ‘petaloka’ may indicate that petas could be thought of as living in the same world as us, just in another ‘visaya’, i.e. ‘neighbourhood’, ‘region’… or I may be reading too much into the wording. Maybe a better rendering of yoni in that case would be ‘condition’, as in ‘human / animal / peta condition’? Or is the semantic of origination too strong to discard?
As for the meaning of ‘paraloka’, my first guess would be it means just ‘the Hereafter, life after this one’, with the exact meaning as hard to pin down as it is in the Christian or Muslim radition. Just imagine the scholars of the distant future scratching there heads trying to figure out what ‘the Other Side’ means. Does it imply the Christians imagined the world as being similar to a river? Is this the motive of a World River like Ganges or world stream from many shamanistic traditions showing up again? Or was death considered to be an invisible wall of sorts? Again, it is just a conjecture, any constructive criticism is welcome and even, I’m afraid, required 
Whether it means ‘world’ or ‘worlds’ is easy to determine by its grammatical form in a given context, I think, as the Pali declension is unambiguous in that respect, whereas the citation form is not really helpful. A brief search on SC shows that the world is used in singular and is thus to rendered as ‘other world’. And ‘lokavidu’, is it meant as singular or plural? No idea 
