2.8 inch tft LCD displays / Max Brightness / Startup White Screen / mirror text for Head Up Display Usage
1) set Max Brightness 


You set the brightness of the backlight (usually) using PWM.


  2) change the start up splash screen from being white to black 


That's not a splash screen. That's just the back light being on before the screen is initialised. Just don't turn on the back light until you need it.


  3) how to change settings so that text can be mirrored so when display is reflected into mirror or glass window the text reflects onto the mirror / glass so that it is readable


Either do it in software (tricky) or look at the MADCTL (many have it called that) register in the datasheet.  The library probably won't support it, but it's part of how it deals with display rotation, so you could examine that function and compare it with what's in the datasheet (hint: 180° rotation is actually flipping both the X and Y axis).