Warranty: honestly, considering the loose warranty of a Chiner frame I'd paint the frame and not bother about warranty. If something really bad happens you'll have to be strong to build the case, paint or not! So, just paint it the way you like.
Seat post: Unless you plan to move your seatpost up/down regularly I'd paint it as well. Install the seatpost to determine precisely it's position (moving it up/down to adjust). Then paint it, add some clear coat (2 layers). Install it as previously determined, using carbon friction paste. It will fit, even with the additional layers (just a very tight fit that will scrap the clear coat and maybe the paint inside the seat tube). So, just paint it as well the way you like but make sure to not get too far in the seat tube because it will then be visible.