A signed zip is still dangerous. If the attacker can convince the user to download two files instead of just one, the attacker controls the names of the folders

A signed zip is still dangerous. If the attacker can convince the user to download two files instead of just one, the attacker controls the names of the folders that contains the extracted zip contents (i.e. the name of the zip), so any app that looks at ../foo/bar would still be vulnerable. The Info.plist approach (where the app declares that it has no such relative path dependencies and so should skip gatekeeper path randomization) seems like the best approach for fixing this "issue".