Rox64 said:

If I try VLC, then the video appears corrupted for 1-2 seconds and then it corrects itself, but at the end of the credits (when the original credits finishes and Harmy's custom credits fades in) the audio repeats the last seconds over and over.

I can't really address most of your other problems, because I didn't download the AVCHD. But VLC does this for every .m2ts I've ever opened with it, on any PC I've tried with. Even those straight from (decrypted) blu-rays that work flawlessly when run as a disc.