preview render will remember as many frames as it can fit into the memory you set for that in Preferences/Memory/Picture Viewer for quick playback and cache the rest, don’t know if that is retrievable after a render. Don’t know that the render Queue saves anything.
And rendering a separate pass without the need to render the image is an often heard request (usually in a similar situation) but I would guess that render passes are pulled with magic math from the render data not ‘the render data assembled from the passes’ (since the passes didn’t exist at first – in the olden days).