Do you know the exact model you bought? I have the Eonon GA5150F and dont have those issues.
1 Hissing....mine has zero hissing but I am using it with aftermarket amps and speakers.
2 In my 2000 328i the radio reception is equal to or better than the OEM radio and much better than the Alpine I had in there before this.
3 Probably because I have tinted windows, but I havent had a problem with brightness yet.
4 I do have some light bleed but not anywhere as bad as yours.
5 This part of your post leads me to believe something is wrong with your unit. Mine is pretty much 18-24 seconds every time. (Ive timed it!)
6 Mine ALWAYS comes back to the last used.
Have you tried a factory reset on your unit? It seems you are having some issues that are beyond the normal 'quirks' these units can have.