Hi Gargleblaster.
Your questions, in a sense, are asking for comparisons between apples and oranges. In regard to the question of the large capacity aero engines in the aircraft you fly, they are generally of the low compression variety designed to operate with lower octane fuels. They are, by comparison to the auto engines mentioned, low revving with maximum speed usually not higher than 2750-2850 RPM. By the way, piston engines in aircraft do not operate at max power for most of the time, nor do they operate at max RPM for long periods. During Take-off and climb are the periods when high RPM/high power is used most.
One of the reasons aircraft engines are susceptible to icing is the environment in which they operate. There are several types of icing which can have an impact on engine performance. Impact icing occurs when super cooled droplets impact with the air intake and usually this type of icing is not removable with heat. Throttle icing and/or fuel icing can occur without any visible moisture present. As the air or fuel/air misture passes through the carburettor the pressure drops and so does the temperature within the carburettor. Throttle and fuel icing will occur with temps from say -10 to +20, especially with high humidity and low power settings. (This is one of the reasons it is prudent to select Carby Heat Full and increase RPM regularly during low power descents) Use of full carby heat will usually clear this type of icing. One good rule is "If you need carburettor heat, give it the full heat"
Your question regarding "torque" v's Horsepower is interesting. The important thing is to have RPM available. With a fixed pitch propeller there is no provision to vary RPM other than with the throttle or by changing airspeed. The greater the RPM the greater the thrust, or if you like, the mass air flow. Horsepower and torque, as it applies to aircraft, are very much inter-related.
I am unaware of any carburettor equipped cars with mixture controls. As you probably know aeroplanes can have manual or automatic mixture controls, however the previous post by 411A says it all. Re-jetting is the conventional way with cars, although most would just accept the richer mixture at higher elevations.
As for your question regarding what is different between a VW travelling at 150 KPH v's a Piper flying at 100 MPH I suppose the flipant answer would have to be "about 10 KPH and varying degrees of altitude".
Hope this ramble helps.
Cheers
Old Fella.