The second part of your question, seems confusing.
"2. The documentation states "On EX Series switches except for EX4300 and EX9200 switches, the vlan-tagging and family ethernet-switching statements cannot be configured on the same interface." This suggests that it can be configured together on the EX4300 and EX9200 -- why would I want to do this and what purpose does it serve?"
Some switches allow the configuration of vlan-tagging and flexible-vlantagging statement at the same time. But I don't know what the documentation means.