Maybe you 'know all that stuff' but have just never formalised that knowledge. Maybe you don't!
Here's your first step. Grab some past papers. Book an hour with a teacher to go through your attempt at one of them. Then you'll know what needs doing next.
A lot of folk just go straight into grade V theory, as a gateway to V+ practical. If you've been playing a while, you must already know a fair bit of theory, maybe unaware of that.
It's impossible to say how quickly you'll do it, but 3 or 4 months may be enough. A teacher will be a boon for some of the more intricate parts, but a large majority can be done ...
Yes you can take it, in that you are definitely allowed to. There are no prerequisites for sitting this exam.
I am a music teacher and have put plenty of students in for grade 8 theory. It's quite tough, but worth the effort.
You need to complete a trio sonata (Baroque style 3 part harmony mostly contrapuntal style), you need to fill in the skeleton of a ...