-- FLASHING SCREEN SOLVED --
For what it's worth to anyone with a problematic Waveshare 7"-C HDMI 1024x600 LCD Rev 2.1, mine came in late last night and so, of course, I stayed up to configure and test it out. Life is never so simple. The editing of /boot/config went without a hitch and mine is set (having commented out any overscan and framebuffer entries that I use for other LCDs) with:
hdmi_cvt=1024 600 60 6 0 0 0
The screen booted up fine and displayed correctly, but the backlight kept cycling on and off. The cycles might change in duration and spacing depending on how I was fiddling with it and I soon discovered that if I held it with my finger in a certain place on the back that the screen would stay on indefinitely. Upon VERY close inspection with a loupe and strong light I realized that the 103 resister nearest the upper-right corner (when looking at the back of the LCD board) had a micro fracture in it. To be more clear, the resister that's at the far edge from the HDMI and USB connectors and at about the same level. It looks like a pull-down connected to the ground plane and was probably cracked when it was placed and soldered.
I couldn't quickly dredge up a 103 but I did find some 104's on a defunct CO2 detector/alarm, so I parallel soldered one of those to the 103. (If you can't solder micro parts then get some help.) After a day now the screen has been working beautifully and consistently, and hopefully we will all live happily ever after.
PS. I initially skyped Waveshare but they're on China time and I'm impatient and don't speak the lingo. I'll probably leave a critical review on Amazon (where I purchased it) as the display was obviously not QA'd before shipping, but I DO like the display now that it's working. Here's contact info if you need it:
(apply for distributor) firstname.lastname@example.org
Address: Waveshare Electronics
Rm 813, Dynamic World Building, Zhenhua Rd, Futian District, Shenzhen, China