I've searched around but can't find an answer -- why do some countries that are fielding teams also have riders competing as individuals? Filling spaces not used by countries that can't find any riders? Just curious
It was based on FEI rankings of individuals who were not selected for teams... Basically, non-team riders at the top of the FEI world rankings list and the highest ranked riders from each region of the world.
Correct. Additionally, some additional places opened up (which were allocated the same way) when several countries who qualified teams at regional games were not able to field teams (because each actual rider on the team had to earn a certificate of capability at GP).