What blutach meant is that you don't need DVD Shrink to backup a dual-layer DVD to a dual-layer 8.5GB blank. Now, if you want to backup your dual-layer DVD to a single-layer 4.7GB blank then yes, you do need to use DVD Shrink
As for the DVD+RW problem, try this: open ImgBurn, switch to Write mode, insert your DVD+RW, click the Erase Disc button (looks like a disc with a red eraser on it) and then click Full. It'll take a while, but doing this will fully format your DVD+RW. By the way, you don't need to erase your DVD+RW before rewriting on it again: just answer yes when ImgBurn asks if you want to overwrite it. That way you won't need to format it again
Having followed your instructions, I heard “Oh, no.” Then the formatting failed at 62%. It still displayed “Ready (Disc Needs Formatting).” The ImgBurn seems having problems to write a DVD RW. FYI, by testing, there is no problem to format and overwrite the same DVD RW with the Nero.