How to ensure equitable distribution of SQS messages in a distributed systems setting?
If there are 100 messages in the queue and 5 consumers, the initial distribution will be no more than 10-10-10-10-10. 

A single response can never return more than 10 messages.  

This seems like a non-issue.  

Race conditions related to multiple consumers should be a non-issue as well.  SQS is designed for multiple simultaneous consumers.

Use long polls and a 20 second max wait timer and be amazed. (No, a 20 second wait does not delay messages by 20 seconds.  It doesn't delay them at all.  You sort of need to see it in action to really understand how it works.)   

You're definitely overthinking some things, I suspect.