Do you get it when not using override relative time? Does it make a difference changing it to 5m? Please refer to How To Use Grafana's Query Inspector to troubleshoot issues and include the complete request/response from query inspector.
Have a look at time series aggregations documentation for table panel.
Not really sure what you mean. The max is per given time unit if that makes sense, You group by date histogram which result in several max values per time unit. I would sugugest you to refer to How To Use Grafana's Query Inspector to troubleshoot issues and inspect the data coming back. That will hopefully give you a greater understanding. Or use the same query in a graph panel.
Marcus