Tags are a way (along with folders) of helping your organise and label your media.
No - all plans include extensive branding options.
You can delete your instance or change the plan it's on anytime via MyMalcolm
You can change the visibility of an individual FAQ within that FAQ's Configure section
You can delete your instances via MyMalcolm or ask the owner of any instances you are a member of to remove you.
MyMalcolm is the control panel of Malcolm! which you can access at https://my.malcolm.support/