I have merged 9 images sucessfully, however I straighten 2 of images and when merged it did'nt use the straighten image but used the original. I have straighted the images in differant software and then imported them to lightroom and everthing is fine
Merging uses the original data, to allow for maximum post processing corrections. It has never been based on uising the edits already applied.
In your specific case, I can see the argument for it, but the only solution I can see would be to create a new original for those specific images either using an external tool like you have, or expoting a TIFF from Lr and then proceed to re-import the image.