Oh wait, I’m not sure I directly answered your questions with my ranting.
ST cannot currently directly control Alexa to speak a notification.
There are no Smartapps that I know that can force Alexa to speak other than perhaps some TTS interface that is converted to “voice” and fed into Alexa’s engine. Maybe EchoSistant can do this? @bamarayne?
All of my workarounds simply use the Echo as a Bluetooth receiver and use the TTS engine of the device that receives the notification from ST. So the voice of my home is not Alexa’s voice.