When we do our branching and merging with svn, we always have the same thing happen.
We get a latest copy of the trunk.
Create a branch and switch.
Personally, I think mergeinfo causes more problems than it solves, at least with the current state of the tooling. Once it's transparent on commits, maybe it will be worth using.
You can safely delete mergeinfo properties. However, be aware that you will need to manually track which revisions to merge for re-integrations.