You are welcome.
Question #1: If I have to design an SD card reader which plugs into the computers USB port, is it the USB OTG circuitry below which is relevant for my use, since the computer is going to act as the host.
A: Yes, you may refer to the schematic,
Question #2: Could you please describe what does the USB POWER circuitry do (in the attached schematic)?
A: When USB_OTG1_ID is high. Beacause of the role of 2SK3018 MOSFET. The EN interface of NX5P3090UK is low. And then there is not output of 5V_USB_OTG. So the RT1050EVKB is "DEVICE".
When USB OTG To Host is connected to the RT1050EVKB , the USB OTG1 ID is automatically switched to a low level, So the EN interface of the NX5P3090UK is high. At this time, there is a 5V_USB_OTG output voltage, so the RT1050EVKB can be used as“HOST” .
Hope it help you.