I know it is difficult, but try to relax. The inteviewers are not trying to catch you out. It in in their (financial) interest if you succeed! They are simply trying to assess whether you have the potential to succeed - no reputable training organisation accepts people they think might fail. So, try to bear in mind that, in the unlikely event of your being unsuccessful, that also has a positive side - you are not at risk of wasting a lot of money on a course you might well fail.
Don't try cramming grammar rules now. You will be taught how to explain grammar once you are on the course. They only want to know if you can spot mistakes and have some sort of explanation of them. If you were perfect, you wouldn't need the course. If you don't know the answer to a question they ask, say so. That will do far less harm than spouting a lot of rubbish. If you are not clear about what they are asking you, say so.
The writing task will probably be moderately straightforward - they want to check that your written English is at a level appropriate for someone who is going to teach the language. If it is about some aspect of teaching, don't try to be too clever; you're not a teacher yet.
Good luck. Let us know how it goes.