Ouch. I sympathize, having gone through similar hoops with Renesas. We buy a hardware product from them and try to develop on it but they won't share more than a few superficial datasheets with us. And I know they have way more manuals / datasheets because they'll sometimes drip the info to me when I ask specific questions, but they won't just give us them so we can do it ourselves.

This is a common business model sadly where the seller wants the buyer to buy an additional support contract for any actual firmware development.