1. The agent, even in infrastructure mode, will go into far greater detail and provide more metrics, for example the AWS integration does not pull in memory usage for EC2 instances. The below links provide full lists of available metrics:
2. Yes, they can also capture metrics across different regions or can be configured to only target the region they're in.
3. Yes, by default Dynatrace will try and split the load across available Activegates and if one of them fails, it will pass the monitoring configuration to another available one. If you spread them across AZs it will still work just fine and you'll get HA.