single-user mode is different on EX3400 and EX2300; please see following KB: https://kb.juniper.net/InfoCenter/index?page=content&id=KB31403&actp=METADATA
aren't you able to boot from the OAM partition?
I don't know if the following page helps you: http://nextheader.net/2017/02/23/snapshots-on-ex2300ex3400/