Can you use the 5* or a color label filter to separate a select number of frames that you already have in LR & LRT3 If you want to redo just a few seconds. I don't want to process all 600 images if I only need 2 to 3 secs of the entire sequence. I want to completely redo the development as well. LR wont let your re-import them- recognize them if you just try and re import the selects. I guess I could just do a new Catalog though

You could edit the sequence normally and then just select those images that you want to export before launching the export/rendering.