Quick question: can I remove the grav logo and grav name in the header and also the „ grav was with by Trilby Media, LLC“ in the footer or is this against any license / usage rule?
Grav itself is MIT-licensed, so you can do what you want with it. This is most often the case for all official themes by the Grav team, later dubbed Trilby Media, LLC. You should check the source-page for your theme, as it will be on GitHub.com and include a license.