Can the intelligence message of an FM signal be cross-modulated to another center frequency due to non-linearities?
Yes and no,   it can't be intermodded onto another SIGNAL, but it can be intermodded onto another CHANNEL.  For example, if you are monitoring 144 MHz, an FM signal on 145 MHz and on 146 MHz CAN, due to non-linearity, end up being heard on 144 MHz.  But it can't be modulated onto an existing signal on 144 MHz.
Mark