Also, while the "strongest spirit" is almost definitely the shadow spirit Spender is a medium for (or it's Lucifer, with him becoming Spender's tool being interpreted as a sign that Spender "defeated" him, I don't think that's the grudge mentioned here. Given what we do know, I'd think it's actually the scrap dragon, given that we know it is a grudge, Mayview is apparently the general repository of spirit stuff, and Boss Leader seemed very interested in having the old snake sliced up and put into her custody.
The shadow spirit being the mystery grudge is perfectly likely, but I think it being Max's spirit is more probable.
Do you guys think the guy in the old Activity Club picture is Mr. Burger or Spender's father/uncle/etc.? He seems to have light hair and the right physique by anime standards (tall, lanky with glasses) to be either one.