1- Yes, you can use one hard drive in WD My Cloud EX4 initially in JBOD RAID configuration but if you reconfigure the RAID afterwards then it would erase all the data as RAID reconfiguration is data destructive process.
You can only Migrate from RAID 1 to RAID 5 by adding drives to a My Cloud without data loss. For more information please refer to the link given below.
2- Data transfer speed depends on the speed of the internet.
3- The maximum capacity of the internal drive that you can use in the My Cloud EX4 is 6TB. We have not tested more than 6 TB internal drive to use in EX4.