It's not a mistake, amixer just tells you what *controls* are available. Just because there's no control to change the volume for the left and right channel separately doesn't mean there can't be two channels.

On my laptop, amixer also says the master control is mono, but that just means you can't adjust the left and right channel separately. The sound itself is stereo .

If you have a multimeter, try to measure the actual voltage on the pi and the usb hub. I had issues with my usb mouse and keyboard even when the pi was working fine, because the voltage was 4.5V instead of 5V (in my case the micro usb cable I was using to power the pi was to blame, but it could also...

You can't just copy the file to an SD card, it doesn't work like that. Read this:
http://elinux.org/RPi_Easy_SD_Card_Setup

I'm not the first one who encountered problems because of a bad micro usb cable, and I probably won't be the last one either, so I thought I'd better tell others about this: the micro usb cable I got from The Pi Hut is completely useless - the voltage drop is just way too high. I assume this is unin...

a push-button linked to +5v pin and a gpio pin

GPIO pins are 3.3V IIRC, so whatever you decide to do, use the 3.3V rail, not the 5V one .

I'm using my pi with a €140 screen, at 1920x1080, and it runs just fine. So I don't think the price of your screen is the problem .