How do I manually install an app in iTunes 12.7?
Have you tried the Apple Configurator 2?

I just read about it here, but haven't actually tried it myself:
How do I download an iOS App (IPA) file to my Mac after iTunes 12.7 update?

Here's a description from the above link:
http://krypted.com/apple-configurator/upgrade-devices-and-apps-using-apple-configurator-2/