MySQL starting 2 processes that can't be used
The name of the first one is join_buffer_size.  And 256K (not M) has been the default since 5.6.6.  Using 256M is likely to blow out RAM all too easily.  This would either slow down mysqld (due to swapping) or crash it (out of memory).

I don't know about the .so, but I would certainly suspect it.

As for 2 processes -- On some Operating systems, several processes, the number based on some configuration values, probably thread_cache_size and max_connections, plus usage patterns.