Not able to access internal memory after setting SD as default

Question

My phone is Huawei Mate 8 (NXT-L29), latest version of Android Marshmallow and Emotion UI, not rooted, bootloader locked.
Today I plugged in a Samsung 128GB level 10 microSD card I got from Amazon. Everything works, speed is good and I loaded some movies on it with my PC. I then switched default storage from Internal Memory to Samsung SD Card in the phone settings. I managed to move my photos and music to the SD Card using Huawei File Manager. Now, every third party app I try (including PPSSPP, ES File Manager Pro, EMUI Theme Editor, Debian noroot for Android and more) only detect the SD card as media (even if the most apps themselves actually are in the Internal Storage along with their data). With ES File Manager Pro, going in either /storage or /storage/emulated I only find the respectively the SD Card as a folder and nothing in emulated. Why is this happening? I still have some files (including Minecraft PE saves) that I'd like to open (not copy) directly from Internal Memory. The only app seeing my Phone Internal Storage is the Huawei pre-installed File Manager. How do I mount/access Phone Internal Memory without rooting?

— Manchineel | Sources

Answer

I am answering to my own question.
First of all, on Huawei, SD Card support is very limited. You can barely use an SD to store files, to save camera pictures, to store some app data in limited circumstances. Setting default media as SD Card does do not much of a change either. Since Huawei is not very open, they don't want you to mess with your phone's data, so if there is a memory card for data storage as an alternative, it prefers dedicating internal memory (the data.img file in the System Flash) to app data, which shall not be accessible by other apps, only by the app that is using the data. I think this is what Huawei would like as ideal configuration, but since not always you can use an SD Card, you also have the "all-accessible" mode.

Moral:
If you want to use an SD card with Huawei (for recording video, storing a great deal of music, having movies) NEVER tinker with default storage.
After setting default storage back to "Internal", ES and other apps are displaying Internal Memory as "0" and SD Card as "E43A-A70F".
If device is rooted, you can create the needed symlinks and not even touch Storage settings.

— Manchineel | Sources

Not able to access internal memory after setting SD as default

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