Yes, present avatars can channel previous avatars during the avatar state and we’ve seen this like you said during the Winter Solstice and Avatar Day. But, its a well know fact that when they channel past avatars, they’ll take on the past avatar’s appearance: Roku in Winter Solstice and Kyoshi in Avatar Day. During the fight in Sozin’s Comet, all we see is Aang, implying that Aang was in control when he took down Ozai.

New Avatar fans turned this into a running joke and now people believe Aang was not in control during Sozin’s Comet. We’ve seen how brutal Aang can get in the Avatar State episode. I don’t get why everyone is acting how he’s not capable of hurting anyone. He literally buried most people in that Earth Kingdom base for “killing” Katara.