Is it obligatory to repeat a prayer that was combined due to light rain, with the prevailing assumption that the rain was wetting the clothes?

If you are reasonably certain that the rain will drench your clothes, then combining prayers is permissible, and the prayers are valid. This is because reasonable certainty (ghalabat al-dhann) is often acted upon in matters of worship. As Sheikh Ibn Uthaymeen mentioned, reasonable certainty has a role in acts of worship, and many acts of worship are built upon it.