Welcome to the forum, intj.
I'll go through some of your post bit by bit: 'Practical English Usage' by Michael Swan is very helpful indeed. You might also find a student's practice book such as 'How English Works' (with answers) by Michael Swan and Catherine Walter helpful. The test at the beginning will show up your weaknesses and lead you to pages of explanations with exercises for practice. Though it's not my favourite book, Raymond Murphy's 'English Grammar in Use (with answers) also has pages of explanations with practice exercises. It has saved many beginning teachers a lot of embarrassment.Your internet horror is most unfortunate. It must have caused you a lot of stress. However, it seems that this did not affect the interviewer's opinion, so you'll just have to try to forget it.The interview: the horror begins
You did positively respond, but it's possible that you could have given the impression that his explanation was mind-blowing to you. If it was a fairly basic point of grammar, this might not have impressed him.We began with an overview of the course. This went well. Next we discussed two of the grammar questions that I had gotten wrong. He asked me if I knew why they were wrong. The first one I knew. The second one I only had a partial explanation. He explained it to me fully. I said something like "wow that is really interesting." (Note: I positively responded to his feedback.)Two points here:He then asked me to talk about a time I was given feedback that I disagreed with. I told him that I had worked for an international school with appalling standards. My director told me to fabricate test results and to not follow the syllabus. He said the students were too stupid and lazy to learn. I told the director that I would not cheat my students and he could fire me if he liked. Of course, he fired me.
1. You misunderstood a fairly straightforward question. This can happen to all of us when we are under stress, and most interviewers have a degree of understanding, but we shouldn't be surprised if this raises a doubt in the interviewers' mind.
2. It is generally not a good idea to criticise a present/former employer or colleague to a third party. If you have to give a reason for leaving an employer in a situation such as this, something like, "I could not agree to certain things which I regarded as unethical that my employer demanded" is quite enough. Don't forget that your last employer might be a good friend of the interviewer!This could come across as "I want to pass the course so I'll just so as I'm told even if I think it's wrong". Perhaps you could think more about this.He then asked, "but what if you understand it and don't agree with it?" I said that if I didn't agrree there would be no point in arguing, reasonable people can disagree. What is more, I want to take the course to learn the CELTA method, so I would just carry on.
Also, some interviewers might think that if you were really interested in the CELTA, you would have done a little research and discovered that there is no CELTA method as such. Did you visit this site: CELTA (Certificate in Teaching English to Speakers of Other Languages) | Cambridge English, and download the free materials?I am afraid that, from what you have said, I might have written something similar to what the interviewer did had I been in his place. You have more or less admitted the truth of the part I have coloured red. The part I have coloured green is difficult to judge without being present, but I have suggested in my previous comments some points that may have led to this judgement."Cambridge ESOL regulations prohibit us from accepting candidates onto the course who we do not feel will be able to respond to the intensity of the course; in your case, with evidence from your application, we are not confident that you can analyse language appropriately in various teaching contexts, interact appropriately with trainers and respond appropriately to instruction and feedback. "This could be part of the problem. Whether you intend to or not, you give the impression that you need the piece of paper, and are not particularly interested in what is involved in getting it. Course providers like to feel that people apply to take a CELTA course because they want to learn how to become good (beginning) teachers. If you do want to become a good teacher, then you need to find some way of getting this message across to your next interviewer. If you are really interested only in the piece of paper at the end, you may not find it easy to be accepted by any CELTA course-provider.I need to get a CELTA. There just is no other option, other than changing careers.
I am sorry if parts of the above sound negative, but I tried to give some idea of how the interviewer might have reacted to some of the things you said.
I wish you luck in your next application, and hope that some of what I wrote may be of some assistance to you.