Sorry to hear of your frustrations. For the EXOS switches, you'll likely want to get them upgraded to the same version for easier management in the future. I might suggest v15.5.3.4-patch1-5 for them.
I can't speak to the K6 Chassis.
Regarding your login, it sounds like it's possible something may have gotten out of sync. Are you saying you're able to login through
http://support.extremenetworks.com/ and when you go to the "Downloads for BlackDiamond Series, Summit Series,..." page, you're getting the incorrect login message?
If yes, try the password recovery page:
https://secure.extremenetworks.com/recoverpassword.aspx
If that still doesn't work, I'd recommend opening a case with GTAC so they can help get you lined up again. Worst case, they can validate your support entitlements and provide the files via an FTP link while your login woes are sorted out.