The Cambridge ESOL website states these requirements:
Ideally you should:
- have a standard of education equivalent to that required for entry into higher education
- be aged 20 or over
- have a standard of English which will enable you to teach at a range of levels.
Centres may still accept you if you do not have formal qualifications at this level but can demonstrate that you would be likely to complete the course successfully. Some centres may, at their discretion, accept applicants aged between 18 and 20.
- So for the second one, no problem.
- For the third one, it's difficult to know without knowing in more detail your current English proficiency level, but it will be at the discretion of the interviewers as to whether they consider your level high enough.
- For the first one, the key here I think is the sentence which follows the requirements:
Centres may still accept you if you do not have formal qualifications at this level but can demonstrate that you would be likely to complete the course successfully.
Here they're talking about whether you're able to show them, through aptitude and other skills, life experience and so on, that you could successfully complete the course.
So theoretically it's certainly possible.
In terms of taking a refresher course, again it's difficult to say without knowing in more detail your level of proficiency. However, if your forum post is indicative of your writing and grammar more generally, my personal view is that you should be absolutely fine. (You'll probably find that you know as much, if not more, about English grammar than most of the other people on the course). You could concentrate on self study - listen to as much English as possible - movies, internet, radio and so on, study grammar online, etc... You need to do what you feel most comfortable with - if you think you would feel more comfortable and confident with a refresher course, then it is the right thing to do.
A good book to have a look at before (and during) the CELTA is Learning Teaching by Jim Scrivener. It's a very good introduction to teaching methodology and practice.
Hope this helps.