I can't help but feel this is rather naive advice, not because of what it is but because:

> my manager left 9 months ago

If you're going to ask for back pay, ask for nine months and cite this as why. It would also look bad if you accept the back pay and just quit immediately, regardless of how you feel about treating your employer that way (personally, I wouldn't care about doing this if I feel like a cog). That might come back to bite you depending on future employment prospects.