Using old roof footings for a ground-level deck
Using the old footings sounds fine, so long as they've done their settling or were poured off kilter to begin with, it's those new ones I'm not too keen on, that will just sit on the ground. If you are going to use pier blocks, you might as well use the old questionable footings; can't be any worse then just using a rock.

You mentioned frost which means 3~5' down for the footings, at least where I live. For reference and how to do it right (in its entirety): Chicago Deck Code (footings: p. 48 and 49) 

I will admit that it not being attached to the structure and not being a fire escape, you could say meh to some of the more colossal building techniques contained therein, but don't be surprised when it sinks those pier blocks into the ground and ends up looking like your sloped patio.