Which of the Fellowship would have gone ring mad?

[Sparrow]

complete ignorance of his danger

I don't know if it was a complete ignorance of the dangers of the Ring so much as a refusal to believe that he could be overcome by said dangers.

As you pointed out, Squatter, any member of the Fellowship would have eventually succumbed to the temptation of the One Ring. Aragorn, Gandalf and Legolas would have been the hardest for the Ring to corrupt for obvious reasons. Merry, Pippin and Gimli would have been the easiest. My vote would go to Pippin because of his age and his interaction with the palantir, but deep-rooted Dwarven lust for treasure and wealth could well have made Gimli 'crack' before long.

~Sparrow