Or is it necessary to use C++ OpenCV?

No, there is no necessity whatsoever. You can use any interface you like and you think you are good with it (OpenCV offers C, C++, Python interfaces).

For your problem about imwrite() and imread() :

For color images the order channel is normally Blue, Green, Red , this is what imshow() , imread() and imwrite() expect

Quoted from there

After reading the documentation of VideoCapture. I figured out that you can tell VideoCapture, which frame to process next time we call VideoCapture.read() (or VideoCapture.grab()).

The problem is that when you want to read() a frame which is not ready, the VideoCapture object stuck on that frame and never proceed. So you have to force it to start again from the previous frame.

Here is the code

import cv2

cap = cv2.VideoCapture("./out.mp4")
while not cap.isOpened():
    cap = cv2.VideoCapture("./out.mp4")
    cv2.waitKey(1000)
    print "Wait for the header"

pos_frame = cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)
while True:
    flag, frame = cap.read()
    if flag:
        # The frame is ready and already captured
        cv2.imshow('video', frame)
        pos_frame = cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES)
        print str(pos_frame)+" frames"
    else:
        # The next frame is not ready, so we try to read it again
        cap.set(cv2.cv.CV_CAP_PROP_POS_FRAMES, pos_frame-1)
        print "frame is not ready"
        # It is better to wait for a while for the next frame to be ready
        cv2.waitKey(1000)

    if cv2.waitKey(10) == 27:
        break
    if cap.get(cv2.cv.CV_CAP_PROP_POS_FRAMES) == cap.get(cv2.cv.CV_CAP_PROP_FRAME_COUNT):
        # If the number of captured frames is equal to the total number of frames,
        # we stop
        break