Why do you think that FLV files can not be edited? You can even try Replay Media Splitter from the Replay Capture Suite.

I've never used Replay Media Splitter. I use freeware VirtualDub for editing files. You can use FLV plug-in for VirtualDub to open FLV files that use VP6 or h.263 video codec with VirtualDub.

VirtualDub is very powerfull. It has lots of filters for editing videos. And it is one of the fastest programs to compress videos.

If your FLV video uses h.264 codec, you can remux FLV video into MP4 using freeware software

FLV videos can use video codecs other than VP6, h.263, h.264, but most videos use the video codecs mentioned above.

You can also use freeware AviDemux for editing videos. I get mixed results with it.