Harry's not lying because he wants to keep his personas from Draco. He's lying because he thinks there's a chance that Draco would insist on being present when Harry confronted his friends if he knew how badly the encounter with Ron had shaken Harry. That's what's changed. Draco made that decision to let Harry confront them alone when Harry still seemed strong and whole. But right now, Harry is fracturing.

At the same time, he doesn't want to hurt Draco by refusing a direct request. That's always been one of the core problems, as well as one of the reasons he started Metamorphosis in the first place: Harry wants to give people what they want, and sometimes he does evil in order to do it.

And it's a good idea not to trust Lucius!