When portage unmerges a package that contains old version of a library, all packages that require the library break because there is only the newer version. I suggest that the library is moved to somewhere like /lib.old or /usr/lib.old, and removed only after revdep-rebuild. Should be easy to do, I have seen it working in a different distro.
No, you should run revdep-rebuild.
*** This bug has been marked as a duplicate of bug 62207 ***