Did some playing around with it, based on your suggestion. What I found out is that mutually exclusive Internal/External setting is not for mount - its the default location where the backups are created at.
The mount points are actually created by checking the checkboxes and you can mount both internal and external at the same time. If you do that, \sdcard is indeed mounted under \emmc like you said, and \external_sd is mounted under \sdc
Hope this saves time for other people
---------- Post added at 03:37 PM ---------- Previous post was at 03:30 PM ----------
Also my md5 code has been submitted into baseline TWRP. I will add reboot to download mode and release a new update for testing soon. The md5 code should now allow apostrophes in the filename.