I'm sorry but while you are convinced everything is correct, it is not. the nRF App is a utility App designed for system investigation and breaks Bluetooth rules, conventions and protocols to get there. It is possible for BLE to pickup certain things defined for BT Classic, this you could regard as necessary as there are still a lot of pre BLE devices out in the wild and things like BLE smart phones want to connect with them - but very often this does not work, especially now that we have such high security measures on Android.
If you want to have a BLE system, use BLE as it is intended. This is what the BLE extension is doing. I fear we cannot help you because you are not willing to listen. I am very busy with my work currently, so I cannot keep repeating myself. However, I think the information from my colleague should help you a lot.
WriteStrings has always been the most reliable and popular BLE function.
Although not everyone is using Bluetooth, there are a lot of people using App Inventor, yet you are the only one that suspects a fault in this function.