I can partially address #1 & #2 - and hopefully TLLim will spot this and comment on #3.
With regards to #1, it does look like the pine64 display configuration supports a dualhead mode, meaning you will be able to run both the LCD and HDMI at the same time... but other than that... I have no idea how you get it to work. These posts by tkaiser might give you some ideas on how it is configured, and what you can do with the pine64 display settings.
For #2, there is no list at the moment, as the documentation is still in a shocking state, or non-existent at times. However, there are two known issues with the Pine64s RPi headers as far as I'm aware. Pin 7 is currently programmed for driving the backlight of the LCD display, so isn't readily available without some changes in code and recompiling stuff, etc. And the I2C bus has the pullup resistors missing, so you will either need to add pull up resistors to the pine64, or to your RPi hat if it doesn't have any.
With regards to #1, it does look like the pine64 display configuration supports a dualhead mode, meaning you will be able to run both the LCD and HDMI at the same time... but other than that... I have no idea how you get it to work. These posts by tkaiser might give you some ideas on how it is configured, and what you can do with the pine64 display settings.
For #2, there is no list at the moment, as the documentation is still in a shocking state, or non-existent at times. However, there are two known issues with the Pine64s RPi headers as far as I'm aware. Pin 7 is currently programmed for driving the backlight of the LCD display, so isn't readily available without some changes in code and recompiling stuff, etc. And the I2C bus has the pullup resistors missing, so you will either need to add pull up resistors to the pine64, or to your RPi hat if it doesn't have any.