The issues you raise aren't solved by limiting transmit power - they are only solved by actually using Qi standard chargers. If you are using a faulty transmitter it can catch fire at 5W or 10W.
The Qi standard as of 1.1 has the receiver communicate back to the transmitter the amount of power it has received. The transmitter then calculates transmit efficiency based on the amount of power it is outputting. If the efficiency is too low the transmitter will fault with the assumption that it could be heating up a foreign object.
The Qi standard as of 1.1 has the receiver communicate back to the transmitter the amount of power it has received. The transmitter then calculates transmit efficiency based on the amount of power it is outputting. If the efficiency is too low the transmitter will fault with the assumption that it could be heating up a foreign object.