If a signing key expires, do existing apks refuse to install, or does it just mean that new apps can't be signed with that key?
We are recommended to use keys that expire in 25+ years, and Google Play mandates expiry of at least 2033-10-22! However, some people may wish to renew their keys far more frequently than that for security reasons (Moore's law, cracked algorthms, etc.).
If an apk's key has expired, is there any way of installing it (without relying on the developer to re-release)?
If a signing key expires, do existing apks refuse to install, or does it just mean that new apps can't be signed with that key?
jarsigner will refuse to use an expired key, and apk's signed with the old key will be refused by Android on install, but apps already installed will continue to run just fine.
https://developer.android.com/guide/publishing/app-signing.html
If an apk's key has expired, is there any way of installing it (without relying on the developer to re-release)?
Yes, the apk can be resigned with your own certificate.
来源：https://stackoverflow.com/questions/10566884/android-what-happens-when-a-signing-key-expires