Torque is a rotational force. It is a measurement of how hard it is applying force that causes the crankshaft to rotate.
Torque is not a very good unit of measurement for an engine because it does not take into consideration how fast the output shaft rotates. For example, given an engine with 100 lb-ft. of torque, if you put that output through a 10:1 gearbox, the gearbox output has 10x the torque (1000 lb-ft), but rotates 1/10th the speed it did without the gearbox.
Diesel engines operate at very low engine speeds (compared to a gasoline engine), so to make power, they need very high torque. Diesel engines benefit from lower wear from lower engine speeds, but they have very high compression, offsetting that benefit somewhat.
If you take both the output shaft rotation speed and the torque into consideration, you end up measuring power. Power is measured in horsepower (in metric, watts).
Horsepower = torque (ft-lb) * RPM / 5252
Watts = N/m * rads/sec
The above are the same, the latter being metric.
The engine "struggling" to intake air at high engine speed is measured as "volumetric efficiency". It is the ratio of how much air actually fills the cylinder, to, how much air could fill the cylinder, at a given engine speed. It is usually expressed as a percentage.
Higher engine speeds should give you more power, automatically, because more fuel is burnt and more power strokes occur. This is true, but it is largely offset by significantly increased internal engine drag at high speeds.
Consider a racing engine. If it can operate up to very high engine speed, it can stay in a lower gear longer. The lower gear gives them additional torque at the output of the gearbox, in exchange for spinning slower.
Conversely, a transport truck engine can hardly operate very far above 3000 RPM, it would need to up-shift much sooner.
Let's make a hypothetical comparison
100 lb-ft @ 8000 rpm vs 400 lb-ft @ 2000 rpm
Both of the above engines make exactly the same amount of power. In fact, you could make two identically powered output shafts, rotating at the same RPM, if you put the 8000 rpm engine through a 4:1 gear reduction, giving 400 lb-ft on both outputs.
You could also put the 2000 rpm engine's output through a 1:4 ratio. In this case, you would have 100 lb-ft on both.
Yes, low air density (usually caused by high altitudes) will reduce engine output significantly. For this reason, aircraft engines are often fitted with turbochargers, to offset the lack of atmospheric pressure.
Ram air intake
Ram air intakes do have a small effect, but they are often there for psychological reasons. It doesn't have to actually help, it has to look cool, and it has to seem like it helps.