Consumer Rights vs. Original Creditor or Collection Agency ???

I've been involved in a debt dispute for several years, involving
services from a medical clinic which cost around $1000. I paid in
installments and after (according to my records) it was paid off, I
received a letter stating I still owed around $300. I immediately
wrote back, challenging their claim and providing information from my
own records showing I had paid off the account. In response, they
sent me the same letter stating I still owed $300. After a couple of
rounds of this cat and mouse game, I ignored their letters.

At some point, a collection agency contacted me about the account.
However, it didn't seem that the account had been entirely assigned to
the agency. Their letters stated (among other things) "Please contact
my client at (909)..." The payment coupon accompanying their letters
was addressed back to the clinic, not the collection agency.

I responded to the collection agency in the same way as the clinic.
In fact, I even sent a courtesy copy to the clinic. The subsequent
letters from both the collection agency and clinic did not address any
of the issues raised in my letter. They were essentially the same,
original letters requesting payment.

To this day, I have been receiving the same letters from both the
clinic and collection agency, but the account does not show up in my
credit report, and I have not been sued. At this point, I don't know
what else I can do except wait and see if they take me to court.

Here's the kicker: the most recent bills sent by the collection
agency have charged me for interest amounting to over $100. However,
ever since I received the first bill, neither the clinic or collection
agency charged interest, that is, until now.

I understand that collection agencies are governed by the FDCPA and,
in California, the Rosenthal Act. I briefly read through these
statutes and found that the collection agency involved in my case has
violated one or more sections. I won't get into the details, but they
are clear violations.

1. Do I have any rights against the original creditor or just the
collection agency?

2. May both creditor and collection agency send me bills?

3. Is the original creditor governed by the FDCPA and Rosenthal Act?
If not, what area of law is involved here?

4. What action should I take against the collection agency?

5. Does the collection agency have the right to suddenly begin
charging interest?