The reason the tq6 wouldn't load is because it only works on one instance of tqsl. That is the one you requested it from. Even if you are on the same computer if you reinstalled tqsl then you have to install the p12. Everyone should save the certificate from the tqsl cert program and you should always be able to reinstall it. Just highlight the certificate right click save. Should be the first thing you do after installing your cert. I keep a copy on every drive I have. Anytime I reinstall the OS, I have to use the p12. I thought you imaged your HDD and the original tqsl on it.
That's funny, because I have TQSL installed on 3 different computers; Windows, OS X and Linux and have NEVER used the P12. I've just downloaded the certs from LoTW and it has all just worked each and every time. When I reinstall the OS, like I just did with my Windows 7 machine a few weeks ago, it worked just fine as well.
That being said, I DO
always make a p12 backup, but I've never needed to use it.