I was wondering if there is a way to tell if a key has been corrupted (any parity, checksum, redundancy, ...)
Yes... the WIF format key that you should have got from decrypting the .key file (for MultiBit it'll be a "compressed" key so it will start with a "K" or an "L"), contains a checksum...
If you can import that private key into a wallet like Electrum without error, then it's almost guaranteed that the key is not corrupted in any way. Changing even a single character will likely render the entire address invalid.
You can see an example of WIF checksum's in action here:
https://gobittest.appspot.com/PrivateKeyNote that this page is quite old so it is using "uncompressed" keys for it's examples, so the WIF keys start with a "5"... it still works with "compressed" keys and WIFs that start with a "K" or "L", it just gives spurious errors.