I don't think the beginning is slow myself; to me, the "motion" in the beginning is more introverted - it is in Snape, in his mentality that is pretty much still at war with himself; he calms as he finds his salvation, piece by piece, through the story (that's another great aspect of the fic - this process isn't rushed), whereas the unsettling psyche is moreorless transferred to Harry in the second half, and he is a living, moving being and so it may feel like there is more action on the surface, although I think Harry, because of his unfailing faith to this world and the people around him, never get to Snape's level of unrest.
Yes, this is definitely about forgiveness, and some of the lines in this fic are really great things to remember in life - some of Snape's advices aside (his moral code truly answers to no one but himself, and that is much more difficult than to just follow what is okay with social norms) there are certain lines that just hit me so hard - the question of the nature of forgiveness and and the players in that game, is every wrong deed forgivable? and perhaps moreso, who is the one who forgives and who is the one forgiven? I always imagine Snape unable to forgive himself in canon (and it is this frustration that drives him to treat Harry badly - moreso than because Harry was a splitting image of James) and in some way I am happy that JKR did not explicitly deal with it - I like to have room for imagination because she may make it really tacky :).
(Sorry about my horrific grammar in reviews! I hope this still makes sense...)