been accounted for by a reduction in torque and it does not follow that "the MP must reduce,
In the case of a CS prop piston engine governing, the torque and MP indication are directly related. Yes, bigger piston engines do measure torque directly, but for the O-470, MP is your only indication of torque if the prop is governing RPM. Think about a manual turbo engine, again, the RPM will govern, so if you add too much power (open the throttle too much), you're over torquing the engine, and over boosting it. The over boost is indicated to you as MP exceeding the permitted limit [for the altitude]. A wastegate failure can get you there too.
Yes, if you had carb ice, and the application of cart heat melted it out, you'd have a higher MP after carb heat cold, compared to that when you selected it hot. The engine is again able to breath, and make more [full] power.
The carb heat induction air path delibertely does not go through the air filter (in case your filter ices over - 'happened to me once). Correct, there is no ram effect when running carb heat hot, but then at Cessna 182 speeds, there's not much ram effect carb heat cold, through the air filter either. If your air filter is truly clogged, then yes, carb heat hot would seem to result in an increase of MP for the same reasoning.
Remember that the "manifold pressure" is rather the induction vacuum below atmospheric pressure. So although it is reading a larger number, it's actually a lower amount of vacuum. It's a bit counter intuitive.