[Avodah] prozbol

[Eli Turkel]

I need help understanding prozbul. It is based on handing over the
loan to bet din.

1. It works because Shemitta is nowadays only derabban
2. It works because  loans through a bet din are not cancelled.

My questions is
a) is #2 from the Torah then why does it work only at a derabbanan level?
or does it #2 work only as a derabbanan because of hefker bet din hefker
b) Given #2 what exacly is the chiddush of Hillel or did he introduce #2
c) In modern days we do not require a bet din chashuv and even
actually giving it
to a bet din. Many paskin that just having 2 people sign a fprm from
the internet
connected to some bet din is enough. I have seen some that would prefer 3
signing as a bet din of hedyotot
How is all this working when it isn't really being given to a bet din
and even the
borrowers did not know

Thanks

-- 
Eli Turkel