There isn't an editing program that I know that can do all of what you want there. Also, it's worth mentioning the site code of ethics
when it comes to downloaded source. That aside, it's also a bad idea to try to edit with x264 encoded files unless you/the encoder used some very specific settings when compressing the video. You can lose your work in the middle of editing otherwise.
There's a decent list of editing programs
over in General Video. The general recommendation is to try the trial versions of a few different programs to see which one works best for you. If you can't find a FLAC plugin for your program of choice you'll probably end up turning your audio into a .wav file.
Program lag is more a problem with your computer's hardware than it is with the editing software.
One more thing
Read ErMaC & AbsoluteDestiny's Friendly AMV Guides Lovingly Overhauled Largely by Zarxrax
It's a technical read, but will save you a lot of headaches in the end.