They kind of addressed all that in this chapter. For one thing, Sasori likely acquired Kabuto as an assistant without knowing that he was already Orochimaru's assistant. As for the Chuunin stuff... that's what the fake identity Orochimaru gave him was for. I don't know if this fake adoptive father was in on the plan, or why nobody recognized him as previously working for Danzou, but the "eyes glowing red" was likely just creative license by the anime team, as I'm pretty sure the manga version was just him having a badass look in his eyes.