Unfortunately, it seems that you might be confirming that EA4500 also fails being able to go back to stock like the E4200v2. The caution advice I added on the wiki page then also apply to the EA4500 (which is basically the same as E4200v2).
As suggested by @builder, maybe you could try to flash an old version first and flash the stock firmware from there to see if it works. He has done it as he said in his post, using USB boot. I wonder thought if this was just because that old release didn't process the checks performed on newer releases, easily bypassed now using the -F switch (assumption dismissed, see his post below).
Just to add that nobody confirmed what is causing the flash failure using serial console. My guess is firmware code check at stock bootup.