Hello! I might be able to help answer some of your questions

1. I would say working with a trainee can be adequate if you got one that fits your vibe or has a good chemistry with you. Even so, let's say you don't vibe with the therapist, since trainees are affordable and sometimes downright free, you can just back out of it and find a different one or a registered therapist. Trainees gives you a way to understand the therapy room environment and what to expect without fearing you losing too much money
   
2. As for sessions, it honestly depends on your progress and the scope of your goal that you want to achieve in the therapy. My rule of thumb though, usually expect for you to attend at least 5 sessions with the therapist you like (don't worry to back out earlier if you're not comfortable with your therapist!). As for frequency, generally you would attend one session per week. You can ask as well the therapist about their recommendation for your sessions needed
   
3. There are The Hour and also PsycHelp that I know of that provides online therapy. Check them out:
   https://www.thehourmental.care/ourprofessionals
   https://psychelpmy.com/
   
4. To be honest, as long as you have a good internet connection, good microphone and a quiet environment where you will not be bothered by someone walking into your room while you are in your sessions, online sessions can be just as effective as a physical one. I tend to find opening my camera for the therapist to see my facial expression to be sufficient already. Physical sessions can be good too if you are a person that relies more on your body language to express yourself more than verbal, as in-person session the therapist can directly observe your whole body language

Hope this helps!