Dwight asked: How can we balance Abigail's assertiveness, without Nabal's knowledge, with Paul's instructions for wives to submit to their husbands in everything?
In the story you refer to, Abagail went out and spoke to David without her husband knowing. She did this to save his life. Nabal, her husband, had acted foolishly, and David planned to take revenge by killing him.
Abagail learned David was coming and went out to talk David out of what he planned to do. She gave David some very good advice.
First, she encouraged David not to stoop down to Nabal’s level (1 Samuel 25:23-25). She knew she was married to a foolish and hardheaded man, and encouraged David to follow a different path.
Second, she reminded him that he should not take matters into his own hands (1 Samuel 25:26). God had restrained David from taking revenge up to that point, and David should not act in such an uncharacteristic way.
Third, she expressed concern that David not diminish his high calling (1 Samuel 28:28). David had been fighting the Lord’s battles. That was a worthy pursuit. Fighting for his own honor would tarnish his role of fighting for God’s honor.
Fourth, she wisely and courageously challenged David to not play the part of God (1 Samuel 25:29). God was taking care of David, and David need not concern himself with caring for himself. God can take care of all David’s enemies, and He had proven that to David in his encounter with Goliath.
Finally, Abigail encouraged David to not forget his future role (1 Samuel 25:30-31). David had been called to be the king over Israel. Fulfilling that role should not also come with the burden of a guilty conscience for having killed a man in vengeance.
To get to the very heart of your question, the reason I believe Abigail is to be commended instead of condemned is because the Bible commends her.
Notice how she's described in 1 Samuel 25:3:
...and the name of his wife was Abigail. The woman was discerning and beautiful, but the man was harsh and evil in his dealings.
Not only was she beautiful, but the Bible commends her for being discerning. That word means she had outstanding insight. She was wise. Abigail loved God as much as her husband rebelled against God. Abigail loved her people as much as Nabal loved himself. She was spiritually wise and Nabal was spiritually blind.
Abigail was not rebellious. She was a peacemaker. She did not violate any biblical commands or sin against her husband.