If I conquer a battlefield with Draven Audacious, go to 7, then charm an enemy unit to the battlefield to win a combat, do I go to 8 off his combat point?

Yes, you can reach 8 points this way.

According to Rule 448.1.a.1, points earned from sources that are not "Conquer" or "Hold" actions are not subject to the final point restrictions (which require you to have scored on all battlefields that turn). Because Draven, Audacious's ability is a triggered ability that grants a point upon winning a combat, it is not beholden to those restrictions and can be used to reach your 8th point.

As noted in the FAQ regarding Draven, Audacious:

• "Draven, Audacious can get you to 8 points to win the game."
• "Draven can trigger his ability without dealing damage (e.g., if opponent's units are removed via spells during Showdown)."
• "Winning a combat" triggers (including Draven Audacious's ability) occur during the combat special cleanup, which happens before Conqueror scoring occurs.