Hi, silicon case is not a good choice, TPU is better choice. Make sure is branded item so the quality is assured.
Mercury is higher quality type of TPU, u will feel it is solid than normal TPU case.

Anyway, price doesn't mean everything but means something.

If u looking for hard case, Nillkin is a good choice. Check out user feedback to believe

Yes, that could be happened. So need to take out phone/casing and clean it from time to time.
Regardless of soft case or hard case, recommended to clean the case and phone occasionally ^^

dust is everywhere, unavoidable hehe