You can't just remove and replace the middle seal. That section is the quantity adjuster and is very sensitive to any adjustment. What you needed to do is index that section to the bottom section just to get it to a position where it will start after removal.
If you didn't index it, you need to buy a ross tech vag com. Loosen the bolts and try to adjust it back and forth until it starts. Then adjust until the injection quantity as displayed by the vag com software shows about 5. The engine might race or not start because even a tiny movement will result in a big change in how the injection pump works.