Trying to find Russian language something less difficult I study also in Alphadictionary russgrammar. I try to find the link between (за)ня-ть и (за)йм-у. It is said: "In non-syllabic roots 'н' and 'м' are replaced by 'a' before a consonant ending". So, if I suppose that the stem is йм-, it will become in the infinitive, for example, йa- -> я , that is, (за)[н]я-ть. But this "н" was in the stem and has disappered ? I don't find in these lessons, very complete on other hand, no explanation to this case. I find also other verbs similar to this: панять. Not in снять. Since both consonants й and н are weak, may be being both together the first "н", would disappear, if the root would be нйм-, and (за)[н]йм-у would become rightly (за)йм-у and (за)ня-ть would be OK. The problem? That this struggle between consonants is presented only when a stem that ends with... is followed by an ending beginning with... but not in any place of the word.
Perhaps просто it is only an exception between exceptions and nothing can be said, save: "Quod Principi placuit, legis habet vigorem"