You should try to reload the raspberry image.
Or: use another SD card
Or: did you changed any overclocking settings inside of config.txt?
This could be at worst a faulty raspberry!

One way could be:
-power the board by 12V.
-measure on which inductor you get 5V to ground.
-one this point you inject your 5V finally.

From my point of view necessary since it looks much better to have the screen flat to the case surface. Without it looks not nice and on top as usual the screen is protected against scratches.

This should work with KEDEI screen but its better to set to its native resolution in order to save power. Otherwise the screen board needs to convert the resolution internally. For the 3.5 inch KEDEI screen I used this setting: dmi_force_hotplug=1 hdmi_group=2 hdmi_mode=87 hdmi_cvt 480 320 60 6 0 0 ...

During login in cyberduck: use your pi's ip adress instead of "retropie"

I somewhere remember that smb will not work. Try SFTP / SSH instead. Use any ssh client for file transfer or at least the terminal to connect to your gameboy to see if ssh connection works.
And by the way: make sure ssh is enabled in the reteopie settings.

Interesting as well since the last source as not delivering anymore...

I had the same: try to delete browser cache. This helped for me.
Edit: at least on Android Firefox browser. Usually I visit this website mostly by mobile...

Your wiring is ok! The wiring on the picture is very simelar: these guys just make use of the internal bridge inside thise switches from left to right. You can measure this: upper left and right are bridged and lower left and right are bridged in these switches. So you can enter your ground on one s...

In Cura: re-size all tree axis by lets say 1.1 if your result should get bigger. If you own a calipper: measure the actual size of your print. Than measure the need size of your tin. For each axis! Example: printed part: 100mm wide. Mintypi Tin: 90mm wide. Resulting: 90/100= 0.9 You must shrink your...