Java 1.8 works fine to build Android apps on OS X. I’m not sure how you got the impression that downgrading Java would do anything from that post most of which is long out of date (we’ll do something to stop others tripping over it in the future).
What do you see if you run the keystore alias password validation manually using this Terminal command:
jarsigner -tsa http://timestamp.digicert.com -keystore 'PATH-TO-YOUR-KEYSTORE-FILE' -storepass 'YOUR-KEYSTORE-PASSWORD' -keypass 'YOUR-ALIAS-PASSWORD' /Applications/CoronaSDK/Corona\ Simulator.app/Contents/Resources/\_coronatest.jar NAME-OF-YOUR-ALIAS
For example, this is the correct command line for the Android debug keystore:
jarsigner -tsa http://timestamp.digicert.com -keystore /Applications/CoronaSDK/Resource\ Library/Android/debug.keystore -storepass 'android' -keypass 'android' /Applications/CoronaSDK/Corona\ Simulator.app/Contents/Resources/\_coronatest.jar androiddebugkey
If that all looks nominal, you could try removing your Corona Simulator preferences in case something incorrect has been saved:
defaults delete com.coronalabs.Corona\_Simulator
You’ll need to log in again and you’ll be asked to enter any passwords again.