Why do you want to go back to the former IT company instead of continuing your teaching job? Teaching management doesn't seem relevant to tester job.
For the first and the second question, the answer is yes, but it's tough because you leave so long. If you were a top performer at the IT company and left for personal reason, you may have more opportunity to welcome back. A gap year is acceptable in your case. You obviously need to bridge the experience gap yourself by some related courses.
For your fourth question, you can use your teaching exp when you are an in-house coach.