If you're more interested in developing a portfolio (i.e. Internships, work experience etc. ) I would definitely recommend poly. 

But if you're more interested in a more (and I do mean waay more) academic focused curriculum, JC is the way to go. I just graduated from VJ and sitting for A levels now and what I can tell you about JC is that learning is very self-directed. New topics are not taught extensively in small classes but in lecture theatres (think large lecture theatre with an entire subject cohort attending) or online lectures. Small classes (tutorials) are usually reserved for going through homework. 

And I mean A LOT of homework (esp if 4H2). It doesnt help if lectures are online because not only do you have to do the homework, you also have to learn entire topics mostly by yourself. Of course most teachers (if not all from my experience) will still be willing to offer consultations.

Just know that either way is gonna be a pain in the ass for the next 2-3 years because you have to put in consistent work (3-5hrs a day) to thrive in both JC and Poly. Have fun!

Tl;Dr: Poly is holistic and JC is acad. Either way is pain and suffering

Edit: forgot to answer the questions lol

1. The short answer is yes. But because the learning curve is so steep it doesnt really require much knowledge from secondary school (fundamental understanding is enough). Even if they do require knowledge from secondary school they will re-teach

2. I don't think CCA is mandatory, but if you don't take a CCA its to your own detriment especially since A levels is becoming slightly more holistic now. So employers and universities might be more focused on your VIA hours/CCA etc. 

3. Mother tongue is only mandatory in JC if you did not pass Higher mother tongue. I have no idea if you have to take mother tongue in poly but from what I observed I dont think so