Missingno. doesn't evolve at all, but another glitch evolves into the ghost form with a different, glitchy sprite. You're right about everything you said. Missingno. is absolutely harmless, except for in Yellow because it usually crashes the game if you try to encounter it with the Mew glitch. And it isn't leftover dats for Lugia, Marill, or any other gen. 2 Pokèmon.