I'm late getting into this, but I'm dying to get my hands on that last book.
I'm in the Snape is working for the good guys camp. I totally believe that he and Dumbledore worked out what happened in that tower. Dumbledore was already dying because of the potion he drank in that cave and Snape had to make the Unbreakable Vow in order to keep Bellatix's suspicion off of him. By killing Dumbledore he would seem to be a loyal follower of Voldemort and thus keep him on the inside.
I'm thinking Dumbledore left some kind of clue for Harry to find that states that it was planned.
In the tower, Dumbledore made sure Harry couldn't help him, so basically Snape was able to kill him. And his plea of "Severus, please." That to me is not him begging to be spared, it was him begging Snape to do what they agreed on. Both men are powerful wizards, with Snape being a skilled Legilimense. They could very well have communicated briefly without no one being the wiser.
I'm sorry this is the last book. I don't want it to end, but at the same time, I want to know what happens!