What an original premise and so very well done. You had me from the very first scene--yeah, I could so see Petunia freaking out. One of the things that struck me in this twisted scenario was how realistic Harry's unraveling and subsequent desire to become Lily actually was. Snape made a very, well, IC attempt to push him away (Harry knocking until his knuckles bled), but the fact that he finally gave in and why was so very Snape, wasn't it. At first I couldn't imagine what would be so stressful about the brewing time, but you did such a great jobs with those scenes, to the point that I could feel my skin crawl with Harry's impatience and anxiety. The technical description of the brewing was fascinating.
The last confrontation between Harry/Lily and Hermione and Arthur was really sad, but your final scene between Snape and Harry felt really good, as it seemed that Harry was on his way to no longer needing to be split, but would still choose it on his own.
You did a wonderful jobs with the het scenes, Elf. Many slash writers are filled with horror at the very thought of attempting it, but once again, your versatility shined through.
Great job of nailing your genre and prompts! You took something squicky and iffy and 'train-wreck-ish' and made it work!