You are right I did miss when you dropped the Durablock and started using 3M hard board
. If I am understanding where you at now, you have a low area that is running the length of the high center point of hood. And you have highs that you chase all over the hood. The paint stick idea might help stabilize the flexing metal and you may not have to shrink it. For the low area along the ridge, you might try running a strip of masking tape right down the center of the ridge. Then skim that whole line to fill in that low, bridging the low with your spreader. Sand by riding the tape and the high area - sanding down with a hard board with 36 or 40 leaving it little high and then change to 80 to blend in with surrounding area. I wouldn't move up to 220 until you are happy with your leveling process.