I see, so electrum may not work, well since you have a wallet and addresses try this it easy to use and uses wallet like you said you want something that can open it.
Go here: https://github.com/prof7bit/wallet-key-tool/releases and download this exe. It requires java v7.0 so download that at: https://www.java.com/en/download/windows_xpi.jsp. Once you install all this i recommend running it on offline airgap computer. Then run the tool, click the load button and select your core wallet if it has password type it. Then you will get a list of all address in the wallet. Copy your address then get your keys. This should work because it will search just your file.