Access root directory? Assuming that I am not able to start up my phone due to a file that is missing on the file system. Would it be possible to get to my phone's root directory on my computer and to place the missing file?

Maybe by using fastboot?
No.

If you have a custom recovery that supports adb / root, you can boot into it and use adb to remount /system as read-write and push the missing file over.  But otherwise you don't have access to most of the phone.