I want to have Alexa speak a response to an intent, but by manually invoking the Lambda function that contains the Alexa skill code, rather than by speaking the intent directly to the Echo.
Could you, for instance, send a JSON payload that comprises an intent request to the Lambda function somehow (by AWS-SDK or via a rule on an IoT "thing") and expect the Lambda function to execute and the Echo to play the speech response?
No. If you call your Lambda function, whatever calls it will get the return, not the Echo.
What you are asking for is "push notification". There is a very long thread of people requesting this on the ASK forum. It is the most requested feature for the ASK. But Amazon have never indicated they are considering doing this. But, then, it is their policy not to indicate what they are doing anyway.
Personally I do not think they will ever do this. There are too many security and privacy concerns. Some people have created hacks, whereby the run an agent on a computer hooked to their echo by bluetooth. They push a request to the computer and the computer plays a message over the Echo. That's the closest I've seen.
