I... actually could see a case for Shadow.
As mentioned above it's not like Shadow is being deceived into being evil, he knows what the plan is and the only convincing he really needs from Gerald is "we need to make humanity pay"
As for magnificence? I will admit that he isn't the most cunning out there, but I feel like he's a case of a style over substance kind of magnificent. He effortlessly broke out of Prison Island using his abilities, took on all of Team Sonic at once, and he's even shown to have a bit of strategy as seen by how he systemtically incapacitates Team Sonic via jumping them in the dark, one by one. He's an incredibly skilled fighter, has some of the best action scenes in the movie, and he even grills Sonic for the fact that he is basically the exact same as him, something that seems to resonate with the blue blur with how he decided to spare him in the end.
Personally? I think his biggest issue is more how tragic he is. He's obviously shattered by Maria's death, to the point where all it really takes for Gerald to convince him to destroy the world is saying "well they deserve it". But I don't see this as too big of an issue either, he's still badass as hell despite this, his tragedy doesn't really do much to negate that, not to mention unlike Gerald he was able to see the error in his ways and redeem himself, which lessons it enough to where I don't think it's too big of a problem.
Ultimately, I understand why people are hesitant on Shadow, but personally? I do think he is at least worth discussing.