Just for everyone's info:
I brought the phone back to the docomo shop, they said they would open it up and clean it.
If the insides of the case would have been bent (from sitting on it or dropping it) and that would have caused the dust to enter, they would have charged me 5000 JPY (around 50 USD) to repair it. Apparently that wasn't the case.
So they cleaned my lens, and they still put a new case on my phone. Which is great, cause my case was chipped. They also renewed my screen or at least the sony protectors on it, because all microscratches are gone! All for the exact cost of free.
All in all, I got pretty lucky. Just had to live with an LG Optimus LTE for 1 week, damn torture if you come from the xperia z