Thanks pierrehammitt, it looks like I've pulled back from the brink...
referencing this thread "Burn new bootloader in Arduino Mega 2560 R3", and now that my board could at least be communicated with, I just took the standard Arduino IDE route as in that thread. My mega seems to be now mega good! I'll keep an eye on it though for compromised parts, as suggested.
N.B. I achieved it using a different computer, but I don't think that was instrumental in the breakthrough - or at least I can't tell, given that I took a brand new route. By the way, I used the .hex file as supplied by the installation of Flip - but of course, ultimately didn't use it in the final salvation.
Thanks again for the contributions.