Transferring Android App between Developers: Keystore and Upload Keys

I'm replacing an Android app on a client Google Play account that was previously maintained by another developer.

I've got the following things to work with:

• a PEM file with a BEGIN PRIVATE KEY block
• a upload cert (DER), from Google Play
• a deployment cert (DER), from Google Play

Unfortunately, there doesn't seem to be any clear way for me to create a keystore to sign the app for uploading and Google docs are, well, Google docs.

It's not possible to get further information from the previous developer.

I've tried a number of things, most promising of which was keystore -importcert and then -changealias, but that just gets me as far as this error:

com.android.ide.common.signing.KeytoolException: Failed to read key [the name of the key] from store "[the keystore].keystore": trusted certificate entries are not password-protected

So at the moment I'm lost as to how to upload the new app, and I'm looking for some sort of recipe to sort this out.

This question Signing an APK with an upload key provided by Google Play seems to be close, but it never works through how to deal with the private key.

from Transferring Android App between Developers: Keystore and Upload Keys