And this is exactly why I think that Lily was not as good as Severus, yet Slughorn still thinks so. Severus as a child probably didn't look like he would be going anywhere important in life, so he wasn't a real interest to Slughorn. Lily, quite obviously, is very outgoing so, as you said, would catch Slughorns eye. I think she did have some talent, but by no means at Snape's level, when he is making changes to his potion. I can even see him sharing his information with her, if they were friends, thus making her seem even more talented, but that of course is debatable.
Now, whether she was better or worse than Snape is another issue, since Slughorn does have *personal* reasons for not liking people (i.e. Draco), and I think prefers *popular* people. Snape may have had a "gang of Slytherins" he hung out with, but I doubt he was ever popular. Nor does he seem to have a lot of charisma or charm, which I think would also appeal to Slughorn. So, Snape might still be better than Lily at Potions, but Slughorn wouldn't admit it because Lily is a lot more appealing (and she *was* popular).
That is true, I completly agree.
That said, I can see Voldemort trying to get more than one person to help with Potions. He wouldn't want to do it all himself, even if he could, because he probably wouldn't have time, especially for any of the tedious details. Having only one person means you have to rely on them in a certain sense, but if he has two, one of them could replace the other. In other words, I think his lack of trust would make it so that he had two helpers and not just one, so that if one ended up being a traitor, he wouldn't lose his only Potions-maker.