When the original 2m distance was being re-assessed the park did say bringing it down to 1m would mean they could fill all seats on Wickerman versus only 6 seats with 2m rule.
Since then they have released videos and pictures clearly showing empty seats/rows between riders meaning they have rowed back slightly on the original message of filling all seats. That would perhaps explain the Th13teen situation.
Regarding single riders, I’m sure you will be able to go on all the rides, they will just configure the guest arrangement in each train relative to the groups they are in. So single riders will be in their own row, probably with empty rows either side of them too.
Edit: Spinball would probably be ok since you will be wearing masks and sitting back to back, that should be enough to allow a single rider to join a group of two sat behind them.