It is very common for aftermarket charger to add noise on the video signal (RGB, composite or S-video) on old hardware. I experience the same problem when I first start retro gaming. I rigorously tested and selected a good manufacturer to avoid this problem so I can fully guarantee that this one will not alter the video quality.

They are the multi-norm (110V and 230V) so you don't need to use a step-down transformer. I would even advise not using it specifically with these because most of the time it will be directly on top of the step-down transformer which generates heat that would make it thinks it's overheating and could trigger its electronic protection. You just need to choose the plug of your country not the plug of the origin's country of the hardware.

No, you can't damage your device, it has a tolerance in terms of voltage. The original are unregulated, so the voltage indicated on the sticker is not real, it's just an approximation, the real voltage can vary around ±3V depending on the load. Mine are regulated, so the real voltage matches the sticker. After that, this voltage is regulated inside to create a working voltage (5V most of the time).

The current value indicated on it, is the maximum current it can deliver, the console determines what quantity of current it needs. Having slightly more power has no impact and have no risk of damaging the device.