I once saw a funny comment where someone said that the Captain ordering someone to arrest Chief in 4 was as good as ordering them to kill themselves, so maybe it's not out of the question.
They were on Reach during the events of H1, Chief met up with them, they destroyed a Covenant fleet prepared by Truth to attack Earth, remained on Earth during H2 while Chief went off to the other Halo/High Charity, then went to a Forerunner shield world called Onyx under the pretence that there was tech there that could win the war, but really it was more of a plan by Halsey to get every Spartan she could to safety. They didn't make it back until after the events of H3.
The notion that Chief was the last Spartan is something that a lot of people thought, but really didn't come up in the games much, if at all. It was in the instruction manual for H1 (when it was thought to be true), but beyond that seems to be one of those ideas that took on a life of its own.
Both seemed to be holding back a bit, but Locke started the fight with a rifle in his hands, an armour immobiliser in his possession and was willing to draw his pistol once the going got tough, with express orders that if he had to shoot, to shoot to kill. He still lost. Frank once posted on here that comparing them out of armour is like comparing a supernatural monster to an Olympic athlete, so I'd say the not overwhelming advantage applies when both are in armour.
EDIT:
Found the post, makes it sound like out of armour, ten SIVs might have a chance against one SII, so I'd say that's a pretty overwhelming advantage.