Yes, simply plug the keyboard in.
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Any USB Microphone
Most probably yes :D There are two usb Ports that are available behind the console. And check that the usb headset is compatible with the wii. In my own experience, i even use my own usb speaker to hear my music while playing wii sports :D Hope this helps.
Use the USB ports in the back of the wii.
Yes, you can use a mouse and a keyboard as long as they are usb or bluetooth. For usb you can just plug and play, for bluetooth you'll have to pair them before you can use them.
If it is a USB port charger - plug it into the back of your Wii (which should have a USB slot) or plug it into any other USB port such as a laptop & games console!