You absolutely can, but trademarks need to be domain-specific. And the social media platform and the window system don’t have much overlap in their respective domains.
Another window system couldn’t come along and call itself “X”, but a microwave manufacturer very well might be able to.
I would strongly assume Protonmail will be doing this automatically soon, there’s no manual day-to-day verification necessary.
Writing to the Blockchain is difficult and takes processing power, reading from it is absolutely trivial though.