Hi Neil - The custom store is fully custom HTML implementation. We have been using this on two separate apps in the legacy viewer for quite a while now. Happy to discuss specific account information in email, if you wish.
As for the debugging, I followed all steps and can see the application in Google Chrome, however the 'inspect' button is unavailable. We assumed it was due to the fact that we never explicitly set the debbuggingEnabled to true. If it is on by default, what other steps must we take in order for the 'inspect' button to become available?