Ok everything went as planned, but something strange happened while I was doing the old man glitch to duplicate my items. I found a level 160 Aerodactyl fossil Missingno and sent my level 7 normal form Missingno against it. In the process of running away it used Leer on me 3 times. On the fourth attempt to run it used Poison Sting and the game softlocked! What the...?
That's odd. I wonder if Bird-type Pokémon can get poisoned and if the softlock may be related to that.
Your sixth Pokémon being Arbok makes sense as the opposing Fossil/Ghost MissingNo. would have base stats, moves based on the last Pokémon in the party.
Edit: I just got a normal MissingNo. poisoned with Poison Sting so they can get poisoned.