The ending of Re:Zero Season 2 in the light novel is tied to a particular plot development and character arcs. You might want to check reliable fan forums or official announcements for the exact details.
It depends on how much of the story you want to explore. You could start from the volume that directly follows the season 2 finale. But if you want a more comprehensive experience, it might be a good idea to start from the beginning of the arc that season 2 left off.
Yes, indeed. The Re:Zero manga exists and presents the story in a graphical format. It might have some differences from the original source, but it still captures the essence and excitement of the series.
The second season of Re:Zero anime has its conclusion in relation to the light novel at approximately volume 16. This means that the story arcs and character developments that were shown in the second season of the anime have their counterparts and endpoints within that volume of the light novel. However, the light novel is a much more extensive and detailed story, so there are still many elements and future developments that are yet to be explored in the anime, especially as the light novel continues to be written and expand on its own unique narrative.