Given the technical and security hurdles, spending hours searching for a “WhatsApp IPA for iOS 7.1.2 best” might not be the most productive use of your time. Here is a better strategy:
Method 2: Sideloading via Sideloadly or Cydia Impactor (Computer Required)
The App Store will tell you that WhatsApp requires a much newer iOS version. To bypass this, you must download an older, archived version of the app.