i always thought something similar to that... the fact that all dual element dragons slayers get their second element from a DS with a lachrima inside gave me the idea that ds can only absorb and gain control of a second element, if and only when it comes from a lachrima (a dragon lachrima), not the mage itself, this being the reason why natsu can still control laxus lighting(who as a lachima inside) while not capable of continue using the magic from another mage, like totomaru (whose fire magic should be more compatible with a flame DS)...
and i also have the idea that something similar happened when natsu got to use flame god slayer magic... after all, it was only stated that Zancrow obviously didn't learn it from a god, nor Chelia (who learned from a book), so probably a requirement for it, is to have a lachrima inside the body (we don't have enough information on chelia, zancrow, nor orga to confirm or denied whether or not they have a lachrima inside), which enabled natsu to gain momentary control over it, being this somehow similar to what happened with ethereon. yet because of the magic coming from a different kind of lachrima, or probably the difference in the very basics of the magic (DS magic- GS magic, DS-etherion highly concentrated magic), didn't allowed him to continue using it.
i do like the theory of of the dragon lachrima coming from a slain dragon, i never thought about it, but it could explain why they are so rare. but i don't think the DS get to form a lachrima inside their bodies, they just get accustomed the the new kind of magic, and get to generate it... that or they have it somewhere inside their stomach, (next to their primary element).. after all the whole body of a dragons slayer seem a bit like a lachrima (generating, storing, multiplying, adsorbing and overall controlling a specific magic). but who knows maybe you're right on that one.