I definitely agree with Hanni- I think your excerpt sounds great!
Hmmm. I guess it would depend on whether Snape was already being used by Voldy during the Marauder era. Perhaps Voldy tries to convert Lily, eventually fails for some reason and
then he seeks out Snape and his Potions skills in order to replace her?
I think that's a good point about Lily being good at charms - Ollivander says her wand was perfect for Charms work. And there are some good charms that can be used in a dark fashion - such as "Scourgify". Maybe Lily could teach those to the Death Eaters - broaden their repertoire a bit.
I personally see Voldy needing a really good Potions person, though, because potions are so fiddly and needing precise timing and exact amounts of ingredients. It's not something to be trusted to just anyone, I don't think, and many of the Dark Lord's Death Eaters don't strike me as the patient type who mind sitting around for several weeks keeping a close eye on the Polyjuice Potion.

Hee, hee!
Please let me know how your story turns out!
~Andrea