Great instructions. There had been one step missing: Before removing the motherboard, heat up the waranty seal above one of the screws with a hairdryer and carefulle remove it with a sharp knive. Otherwise it's broken.
On my phone, the switch to turn louder and more silent got bent. Hence, the volume down butten was pressed constantly. Additional to the shown steps to disassemble, I had to heat up the side part and carefulle remove it to get to the switch. After straighten it, heat up again and press firm.
Everything worked fine again.