Well I dislike using Oblivion as a resource for such conversation, but here goes. What of the level of "souls" each creature possesses? If there is such a tight Goblin/Meric link why are their souls trappable outside of Black Soul Gems? Or am I missing something key here?
Goblin souls can be trapped in normal soul gems, but I wouldn't really take the game mechanics regarding soul trapping as a very good indicator of lore, though. As only "humanoids", which in game mechanics terms, means the ten playable races, as well as some unplayable ones like Dremora and Sheogorath, require black soul gems to capture. Yet Dremora could always be captured using normal soul gems in past games. So either Oblivion's soul trapping system is not an entirely accurate representation of how souls actually work in the Elder Scrolls universe, or the past games' is not accurate. Either way, it reminds us that gameplay mechanics don't always reflect lore. We must also consider that in Morrowind, Almalexia and Vivec could both be soul trapped, as could Sxith House creatures and corprus beasts. Even though the Tribunal were originally Chimer before they used the power of the power of the Heart to attain godhood, and the Sixth House monsters are Sixth House priests whose bodies have been transformed by corprus, whereas the corprus beasts are victims of the disease who have become grossly deformed and lost their reason. Did the Tribunal's godhood also stop their souls from being protected? Does corprus disease also have this effect when it transforms the victim too much? Or is this, once again, only gameplay mechanics? If the former is true, than it says nothing about goblins, and if the latter is true, than it may show us that the way soul trapping works is not proof that goblins are not related to elves, as it means that whatever caused them to become the way they are now could also have made their souls able to be trapped in normal soul gems.
But I could certainly believe that goblins have some connection to Malacath, since it seems to be suggested that at least ogres and trolls are somehow connected to him (At the very least, he seems to make it clear in his quest in Oblivion that no one owns ogres but HIM.) and while there isn't really any indication that goblins are related to trolls or ogres, I wouldn't put it past them to be. I doubt Bethesda is going to elaborate on their origins too much, though, they seem to mostly just exist to offer cannon fodder for players to kill, and are used because they're a generic fantasy concept fans of the genre are already familiar with so Bethesda doesn't have to explain anything more about them to know what they are, all we can do is speculate based on what we've seen so far, but maybe I'll be proven wrong some day and Bethesda will decide to give us lots of interesting lore on goblins, and then we'll be discussion all the questions that
didn't get answered, or complain that what we learned isn't consistent with our favorite fan theories.