you should never upgrade packages.
I strongly disagree. Especially on stable branches, package upgrades are supposed to work fine. If they don't, it is a bug that needs to be corrected. In this case the various hostapd backports were improperly done (e.g. no strong dependency version constraints, no check for availability of new features).
@eehmke: you need to upgrade both hostapd-common and the installed wpad flavor simultaneously. The updated scripts in hostapd-common rely on new features in the compiled wpad part which are not available with the older version.