API Help: Burned-in Captions, B-Roll, and Speed Settings (Creator Plan)

Hi HeyGen Team,

I am using the V2 API on a Creator Plan. I need to fix three things in my automated workflow to avoid manual editing.

Here is my current JSON:

JSON

{
"title": "Test Title",
"caption": true,
"video_inputs": [
{
"character": {
"type": "avatar",
"avatar_id": "12345678910", //example id
"avatar_style": "normal",
"motion_engine": "avatar_unlimited"
},
"voice": {
"type": "text",
"input_text": "hi, this is a testing sample",
"voice_id": "12345678910", //example id
"speed": 1.2
}
}
],
"dimension": { "width": 1280, "height": 720 },
"video_settings": { "output_language": "English" }
}
My Questions:

Burned-in Captions: Previously, my videos generated with captions on the screen. Now, "caption": true only gives me a separate SRT file. How do I get the captions "burned" into the video frames using this API?

B-Roll: How can I add B-roll images or videos directly into this request? I need the visual assets baked into the final render.

Voice Speed: I want my voice speed at 1.2. However, the API seems to only accept 0.5 increments (1.0, 1.5, etc.). How can I use a precise 1.2x speed?