[re]mount the internal sdcard on the Huawei U8836D (G500 Pro) please?
Since you are okay with Linux and ADB, let me first clarify 2 things:


Repartitioning the internal SD for Link2SD specifically as per the guide will partition it with a filesystem like ext2/ext3/ext4, which will cause partition to be recognized as system and not show up in most file managers. Some file managers, like ES File Explorer, may allow you to access it through the /mnt namespace, if it was mounted, or as some block device in /dev if not. Link2SD on first startup detects this partition by running some variant of the mount command and adding a mount-at-boot for this partition to init.rc and/or vold.fstab, and makes its permissions so that it isn't easily accessible even by the system.
This not showing up is intended behaviour, but you will find the partition under Link2SD's partition options as "SD card Partition 2" or some such.


Now, if you want to go back to using that internal storage for something, you would have to repartition the internal SD again, using GParted or some such software or Linux which recognizes an ext filesystem.

You will have to:


Find the correct partition for Link2SD.
Format it to FAT32, and set it as active and primary.
On device, mount the partition's block device (mmcblk0p6 in your case) as /mnt/sdcard0 and add this in init.rcand/or vold.fstab (Follow this and this answer).


For you the command to enter in vold.fstab will be

dev_mount sdcard /mnt/sdcard auto SDCARD=/dev/mmcblk0p6

And reboot. It should work. Format this position to FAT32, not VFAT, to avoid useless errors about "read only filesystem".

I'm not providing the commands here as you are familiar with Linux, and in this case on Android it's similar (a root shell through ADB).