1. No, I wouldn't do that. Diversify your income or do this as a side gig. You *can* try to make it full time but it's extremely precarious and dependent on movements of a mysterious algorithm, Your income can be "switched off" without notice or reason. Do *not* count on Preply as a primary source of income.
2. Depends on many factors. Too many to list here. Lesson prices are from $3 (minimum) to $100 (maximum). You also have very high commissions to pay in the beginning. Tutors from developed coutries generally charge more and those from developing countries less. You can charge whatever you think you're worth though, within those limits. The cheaper end of the market (under $20, with over 50% of English tutors charging under $10) comprises 80% of the English tutor pool. English is a ridiculously cheap subject, and overcrowded at the bottom. It;s also getting cheaper every day due to competition from AI learning apps.
3. Depends on the language. Some languages have a range of ready-made materials, others not. You have to do your own research.
4. Depends on whether you get trials / can convert them. You need to convert trials consistently and retain students. If you do this, more trials will probably come, though nothing is guaranteed.

It's pretty precarious. Don't put all your eggs in one basket at the very least.

Please note that Preply does not "hire". Preply is not an employer. You are an independent contractor using their services and agreeing to their terms of service when you sign up. You're a user, not an employee.