1. How are the chances I can get the domain because of invalid Whois-details

Whois info is required to be valid. Maybe you can try to find the owner's correct email.

2. or on the argument that they're not using it

It matters more how they are using it. Does the domain resolve to any website or landing page?

If you really want the domain, you can try to hunt down the correct email or phone number to try reaching the owner and negotiate a purchase.