You are right that Bingley is using awful to describe Darcy as solemnly impressive; awe inspiring (#3) and this makes his speech about paying him deference much, much clearer to me. Bingley is poking fun at Darcy and Darcy smiles because he knows Bingley is describing his demeanor well but that is not the reason Bingley relies on his judgment. Prompted by your activity, I looked to my copy of Samuel Johnson’s Dictionary, A Modern Selection by E.L McAdam Jr. & George Milne and this is the only definition for awful listed:

Awful: That which strikes with awe, or fills with reverence.

I do think this description of Darcy is to add to the explanation for Darcy’s behavior at the assembly but not to excuse it. Darcy’s countenance may be awe inspiring, especially when he has nothing to do but he was still uncaring how he treated people because they were beneath him and I do believe he understood what was expected of him socially at the assembly and did not do it for that reason. His treatment of Lizzy is of course unpardonable no matter his state of mind. I read the description of Darcy being awful, even with the new meaning applied, as Darcy was not particularly pleasant on a Sunday evening with nothing to do at Pemberley but rather unusually solemn and distant—perhaps awe inspiring but not congenial. Bingley’s speech indicates to me he understands his friend’s temperament but likes him in spite of it for other attributes not always readily apparent. ;D