If you are going to test it with a different Electrum version I suggest you make a copy of the wallet.dat before you import it to any version of Electrum.
I think Electrum doesn't generate a wallet.dat file it usually generates a wallet file with no extension.
So it should be work on BitcoinQT just make sure to make an original copy of wallet.dat before you use any tool to decrypt and extract the private key.
Why not try to install Bitcoin core instead and replace the new wallet.dat with your old wallet.dat file and maybe your old addresses might show up on the addresses tab.
Or if you know the exact address of your wallet you can go to the console tab to dump the private key of a single address.
Here's the sample command
Code:
dumpprivkey "yourbitcoinaddress"
Remove double quote after you replace it with your BTC address and if the wallet.dat file is encrypted with a passphrase do this command first.
Code:
walletpassphrase mypassword 60
60 is the time limit in seconds. After that apply the code above with the exact address the result should be your private key in WIF format. You can import it to any wallet like Electrum.
