For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
You should test with USB macOS pendrive, with Link LED.
If LED flashes, there is 100% USB activity.
If not, no real reason to search for video; board doesn't really boot.