1. I think disable_parallel_leader_participation related code can be extracted into a separate patch as it is mainly a debug/test aid. You can also fix the problem reported by Mahendra in that context.
2. I think if we cam somehow disallow very small indexes to use parallel workers, then it will be better. Can we use min_parallel_index_scan_size to decide whether a particular index can participate in a parallel vacuum?
Forgot one minor point. Please run pgindent on all the patches.