I have always used a path to a local APK in my Appium testing so I am by no means an expert here but… It looks like the revolves around finding the MainActivity in your app. Are you absolutely sure that the app’s launch activity is actually called MainActivity? Its kinda rare but I have seen Android apps that dont launch an activity called MainActivity and instaed use something like “HomeActivity”. I would also see if there is any success in specifying an “appPackage” but not setting “appActivity”… Its a combination I dont see in what you have tried already and Appium may be able to lock onto the package and then detect whatever activity is actually running.
Another thing to try is to isolate out Appium and make sure you can just launch the app with Android tools. A command line call to ADB like the following should start the app and activity if it exists
adb shell am start -n com.example.app/com.example.app.MainActivity