I think your trying to make things more complicated for yourself than what they really are, of course it helps to know a little Linux when doing things like this.
1. Will the command line entries automatically be able to write to /system/xbin/?
Yes, that's what the mount -o remount,rw /system does in the command you run.
2. If yes, will I then be able to read and write to /system/xbin/ using ES File Explorer?
No, this is not needed as the commands handle this for you.
3. If not, how do I gain access to /system/xbin/?
You don't... just run the commands for rooting via the terminal and everything is done for you.
The ONLY thing you need to do with ES is to extract the downloaded su file and put the su file itself in /mnt/sdcard. Once you've done that, open the terminal emulator and run those 3 commands.
HTC Aria - CM 7.1 / Rooted / OC'd - 801mhz
Motorola Atrix - CM 10.1 / Rooted / OC'd - 1.2ghz
LG Optimus G - SnowJB / Rooted :: (Daily Driver)
EKEN M001 - Android 1.6 / Rooted / OC'd 533mhz
Acer Icona Tab A100 - Stock / Rooted / UC'd 600mhz
Yuandao Window N70S - Stock (for now) :: (Daily Driver)