Yes it's usable by everyone. It's secure by default but anyone can make something insecure. For example granting malicious apps accessiblity permissions would not be great for security.

It has 99.99% android app compatibility. Over 90% of banking and government apps work. These apps take extra measures to ban grapheneos, apps must put in work to make their app incompatible, not the other way around.

I wouldn't say anyone can use it, if you can't sign in to a Google account by yourself then you would have trouble setting it up. But that would be similar on iOS. For the average person, definitely. There's no code or anything like that. Works just like stock Pixels.

If I was giving it to my grandma then I would install her apps and she would be fine clicking icons. But similar on iOS.

Yeah that RAM mention is very strange, not the best article.