I met the same E160004 error, and I found the following two links helpful:
https://subversion.apache.org/docs/release-notes/1.8.html#verify-issue4129
So as for your Question 1, which I think you meant that you want to know how to fix this issue, "perform a dump/load cycle", as suggested in the second link above.
Question 2, it seems impossible, as I read the help of svnadmin and tried to do that.