The Titan security chip is not a black box. The Titan M1 gas been scrutinazed by blackhat: https://dl.acm.org/doi/fullHtml/10.1145/3503921.3503922
Just because something is not open source does not mean you can't verify it (no, i'm not shilling closed slurce; no i don't think closed > open; no i don't think closed source is more secure)
What do you mean ? This has nothing to do with GrapheneOS in the first place (which by the way has been created in 2014. The article i linked refers to 2021).
Reverse engineering is a thing. It always has been. If every piece of closed source was a blackbox how can you explain exploitation ? How can bad actors exploit Windows, MacOS, CPU firmware and so on ? Your argument here is not practical. Also, why should Google put a backdoor inside a chip ? They already get every information they what directly from the people agreeing to use their software. So, why bother ? Moreover, every phone on the market has closed source firmware.