RG406V button setup and shaders question

Trying to setup RG406V with ES-DE. Have couple of questions:

1. Is it possible to use central round button for retroarch hotkeys? I can choose it in hotkey settings but default function is "open retroarch" and overlap occurs.
   I can use "select" button for hotkeys but it is not optimal - some games use it too and it is placed very low on RG406V.
2. How can i clear button function in retroarch? Tap, hold, all buttons on the device - nothing works.
3. Shaders. When i press "Load preset" there is menu with folder paths appears:

/storage/emulated/0

/storage etc.
Do retroarch always have preinstalled shaders? Can i find them somewhere? If not how can i download them *without* online updater? Updater dousn't download anything in my country even vith VPN (but it works fine on my phone with same internet connection).