My plug application is not so hungry for processing power. Price and power efficiency are more important.
I would sort the points from higher to lower as follows:

9- Must be below $100
8- Must have the best power efficiency
4- Must be quite and doesn't include any moving parts
7- Must have the smallest footprint possible.
6- Must have the highest processing power available (vs. lowest price/performance).
3- Must be packaged within an enclosure (vs.  bare-bones)

Personally I would go for a DC power input (maybe with optional AC/DC  plugONplug) to reduce the overheat issue.
Having PLC capabilities and a digital audio interface (or combination of both) would be a nice upgrade for the device.

Try using the attached binary files for this.

The same flow is used for both "Boot from UART" and "Debug prompt". The flow should be as follows:
- Hold the board in reset.
- Start sending the appropriate file (either debug or boot) as plain binary (no Xmodem or any other protocol)
- Immediately release the reset (so as to execute the BootROM while the pattern is still being transmitted).
- Wait for BootROM prompt (if debug file used) or for NACK signals (if boot file used) and if you terminal shows these NACKs.
- If you are doing boot. Start sending the UBoot with Xmodem.

I suggest to start with the debug prompt first, since you have a visual indication.
I am using the attached files and I can successful do both debug and boot. It is a matter of timing...
If you don't succeed in this try to enlarge the size of the pattern file, this will enhance the BootROM chance to catch the pattern...

I hope this helps.