Unable to install Sierra

I am trying to do a clean install of Sierra to a newly purchased SSD.

Since the Sierra installer is no longer available in the App Store, I'm using an installer I saved back in November 2016 (version 12.0.57).

The installer stays stuck at "About 7 minutes remaining". I found this in the logs:

(com.apple.install.osishelperd[31435]): Binary is improperly signed.

Dec 10 09:41:20 eric osinstallersetupd[31427]: objc[31427]: Class OSISProductManager is implemented in both /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/OSInstallerSetupInternal.framework/Versions/A/OSInstallerSetupInternal (0x1066f0390) and /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/OSInstallerSetupInternal.framework/Versions/A/PlugIns/OSInstallerCompatibility.bundle/Contents/MacOS/OSInstallerCompatibility (0x1067c80e8). One of the two will be used. Which one is undefined. Dec 10 09:41:20 eric osinstallersetupd[31427]: objc[31427]: Class IA_PKDownloader is implemented in both /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/OSInstallerSetupInternal.framework/Versions/A/OSInstallerSetupInternal (0x1066f0ae8) and /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/OSInstallerSetupInternal.framework/Versions/A/PlugIns/OSInstallerCompatibility.bundle/Contents/MacOS/OSInstallerCompatibility (0x1067c8160). One of the two will be used. Which one is undefined. Dec 10 09:41:20 eric osinstallersetupd[31427]: objc[31427]: Class IA_PKDownloaderSpeedTracker is implemented in both /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/OSInstallerSetupInternal.framework/Versions/A/OSInstallerSetupInternal (0x1066f0b10) and /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Frameworks/OSInstallerSetupInternal.framework/Versions/A/PlugIns/OSInstallerCompatibility.bundle/Contents/MacOS/OSInstallerCompatibility (0x1067c8188). One of the two will be used. Which one is undefined. Dec 10 09:41:38 eric com.apple.xpc.launchd[1] (com.apple.imfoundation.IMRemoteURLConnectionAgent): Unknown key for integer: _DirtyJetsamMemoryLimit Dec 10 09:41:39 eric com.apple.xpc.launchd[1] (com.apple.install.osishelperd[31435]): Binary is improperly signed. Dec 10 09:41:39 eric com.apple.xpc.launchd[1] (com.apple.install.osishelperd[31435]): removing service since it exited with consistent failure reason When validating /Applications/Install macOS Sierra.app/Contents/Frameworks/OSInstallerSetup.framework/Versions/A/Resources/osishelperd: The platform identifier value 2 is not the minimum expected value 3 for this type of binary and OS release. Dec 10 09:41:39 eric com.apple.xpc.launchd[1] (com.apple.ReportCrash.Root[31436]): Endpoint has been activated through legacy launch(3) APIs. Please switch to XPC or bootstrap_check_in(): com.apple.ReportCrash.DirectoryService

The "improperly signed" message concerns me. Is it possible that Apple is preventing us from installing an older OS, even if we saved the installer?

I could clone the drive, but really would prefer a clean install instead of bringing along years of junk. And no, I'm not ready to switch to High Sierra.