I have some general questions for AWS IOT MQTT. Im new to MQTT and I mostly use AWS for EC2 type stuff. So my questions are fairly general.
My use case is that I will have 100s of devices of the same device type waiting for instructions. These instructions will be a simple numeric value from 1-100.
Questions:

1. Should I have all devices subscribed to the same topic and have some sort of device address in each message sent?
2. I’m using C# to publish messages to the MQTT that is running on a EC2. Should it use something like SNS to publish to MQTT or should I have the C# act as a special thing and have it publish to the MQTT.
3. Does each device need to be its own THING or can I share the key & pem files between all the devices.
4. Is MQTT the right way. I looked at SMS to send this message. The devices do have sim cards but I was not clear on how to get SMS working in AWS. There is talk about 10DLC and short codes and a lot of the documentation is out of date since some SMS rules have taken effect.