OK, I got it working (external sd and internal storage showing up on Windows when Kindle Fire is connected to computer via usb cable). (See screenshot.) I was able to transfer movies from Windows PC to Kindle external SD and watch the movies on the Kindle. I did this as follows:
When the Kindle was connected to PC via usb cable, I didn't see any hidden screen about MTP or camera connection. However, there was a screen in the settings that said something like, "put personal videos and files on sd card". I can't remember exactly what it said, but I ticked that option and then the external SD card showed in Windows. Also I was able to use ES Explorer on the Kindle to play the video that was on the external sd card. Also there were options in the Kindle settings to put downloaded Amazon videos/books/music,etc to the sd card, but I didn't bother ticking those items because as a rule we never buy any Amazon content.
