With that said, it's time to address.....
#6
Saul doesn't David to marry his daughter. He sends David on what is essentially a suicide mission, not expecting him to survive. Keep in mind that Saul is staring at the man he knows will replace him, for his botching up the Amalekite mission. Saul is trying to avoid the inevitable: David will supplant him as king. David, however, might have been trying to marry this woman, to quell the murmurs of Saul loyalists. By marrying into the current royal family, he would ensure that Saul's descendants will continue to rule on the throne of Israel. But, God has rejected Saul. Neither he nor his descendants will rule Israel again.