Boomerang lilacs are called that because they re-bloom later in the summer. Cutting out the dead flowers after this spring's bloom was the right thing to do. You should have also cut out the old wood. But if you cut back those new branches, you are cutting off the rest of the show.
Although Boomerang lilacs are considered dwarf, they still get 5 feet tall and wide. If you want flowers, you are going to have to make some decisions about moving this plant or moving some of its neighbors.