Using Followup Intents is probably the wrong way to do this. As you note, it is way too heavy for more than a few Intents. It may be useful in certain circumstances if you want the "repeated" message to clarify the response in a different way, but in general, it isn't very useful. (It should also be noted that Followup Intents use Contexts, but in a different way than discussed below.)
You don't need to add the Context to the UI as part of the Outgoing Context - you would set this as part of your Fulfillment. It would include a parameter that either contained exactly what you said, or the information you needed to recreate what you said (possibly in a different form, if appropriate). In your "repeat" Intent, you'd read the value that you had saved in this Context, and send it as the output again. If you're using SSML, you may wish to change the speed or volume, if that is appropriate.
Update based on new questions
The readme for VoiceRepeater has the basics of what you need to do to use it, but it does assume a little familiarity with Node. But in general, yes, you install it the way you describe, setup an Intent that captures requests to repeat, and registers a handler function (repeatLastStatement(app) in the readme) that handles the Intent to send a reply through voiceRepeater.lastPromptWithPrefix().
It also may assume you're using version 1 of the actions-on-google library. I haven't dug too deeply into the code, but it looks like it replaces the library's ask function with its own, and I'm not sure how well that works with version 2 of the actions-on-google library.
Unlike Voice Repeater, multivocal doesn't require you to register handlers specifically since it tries to hide as much boilerplate under the covers. You just need to define the replies that you might want it to use. It uses the Context scheme I outline above to store responses and make them available when the user asks for it to be repeated.
There aren't any videos on using multivocal, but the simple example does include the configuration illustrating how to configure responses for the "multivocal.repeat" Intent. While VoiceRepeater works with the actions-on-google library, multivocal is a complete replacement, offering a more template-based approach to building fulfillment.
However, neither of these directly help you if you want to implement it for Python. But if you look at the source for VoiceRepeater, you can get a sense for how to implement it yourself in Python.
The key bit is on line 47 where it saves the reply in a context. (It also saves the reply with a prefix message.) It then calls the original function that would send the reply:
app.setContext("last_prompt", 100,
{
"last_prompt": textToSpeech,
"prefixed_last_prompt": repeatPrefix + lastStatement,
});
originalAsk(response);
Later, in the call to lastPromptWithPrefix(), it uses the contents of the Context to send a reply.
lastPromptWithPrefix() {
return this.app.getContext("last_prompt") !== null
? this.app.getContextArgument("last_prompt", "prefixed_last_prompt").value
: "um....I don't remember what I said!";
}
