The BDM unit arrive today, and I'm happy to report that the DETA 4Gang switches shipped with a later firmware (that had the new PSK) were successfully serial flashed. 
For other newbies like myself who's trying the serial flashing. Here are a couple of my learnings from tonight:
- I had several variety of errors (basically failing at various stages). In the end, I found that my case was due to high impedence. Got rid of the breadboard, any unnecessary jumper-wires helped.
- I used both Mac (with Apple USB-C -> USB-A adapter), and Windows. sparkfun FTDI and CH340G. All combinations gave me random failures, and all worked well when I minimised wires/boards. I didn't have external 3.3V PSU.
- I accidentally had FTDI VCC at 5V, and didn't fry the ESP8266. But PLEASE don't try yourselves.
Thanks again @markus, for the tip. I'm very happy with this jig. Also, you were right that I needed one more pogo stick. It was for the IO0 grounding to get the ESP8266 into programming mode. Also many thanks for your amazing work on the HE integration. Our whole family truely depend on your codes now 
