Also, if you just say you lost your iPhone and cancel the data plan, moving the SIM card to a flip phone, then move the SIM card back into the iPhone, can they find out that you are using the iPhone and forcefully put back the data plan in again?
If "they" = AT&T, then yes. The IMEI identifies it as an iPhone.
I thought you needed to make a contract, can you escape that?
What if they are not AT&T?
I'm not sure about other operators. In this part of the world the device is irrelevant; you get the plan that you choose to pay for, but I can't speak for other American operators.