Yellow. Definitely yellow. Not SCREAMING yellow, but a definite, obvious, "this-is-a-yellow-door-and-I-love-it" Yellow. you know, choose a yellow color that's somewhere between classic bold yellow, and the cream color already on your house... yeah, definitely yellow. :)

No don't do any "pop" on the front door either inside or out.

I have no idea. Light conditions, landscaping, personal taste all play into the decision. The black will look crisp the way your brown does. (I like the brown, which goes well with the stone.) Here are some houses with tan stone entry/walkway, which have black doors, but more glass in them. Do a Goggle search of "white house, black door, tan stone walkway" or the blue, and probably a ton of Pinterest images will pop up. Good luck!

I think a bright green like the color of your tall bush would look great. Or think about what colors you are using inside your home, and coordinate with that.