When a branch is reintegrated to the trunk, is that branch effectively dead?
Can you make modifications to the branch after the reintegration and merge those back
Actually, you need to do a --record-only merge from trunk into your branch of the revision that was created by the --reintegrate commit:
$ cd trunk
$ svn merge --reintegrate ^my-branch
$ svn commit
Committed revision 555.
# This revision is ^^^^ important
And now you record it
$ cd my-branch
$ svn merge --record-only -c 555 ^trunk
$ svn commit
You are happy to keep the branch now
More information is in Chapter 4. Branching and Merging, Advanced Merging.