Yes in the diagram S1 is a spur. You do end up with 3 cables into the S2 back box that can be tight. Hence the choice to use a wago box externally.
Yup. However if the feed from DB goes into S1, which then feeds S2, which then is continued onto the rest of the ring - you could in theory wire a spur from either of those twin outlets depending on whichever looks the neatest for the SWA arrangement. Am I correct with that?