Generally, no.
The force they collide with is equal to the forces of both combined, and each one slows down by the force of the other.
It is due to the momentum of the two bodies.
one air mass is denser than the other (check answer again just in case)
Other things being equal, yes; in general, no. The mass also depends on the density.
In the absence of any other force on them other than the force of gravity, all objects, regardless of their mass, size, shape, density, color, creed, or religious, political, or gender affiliation, fall with the same acceleration. That means that with equal initial velocities, their velocities are all the same after falling for equal times.
The answer depends on what the numbers measure. If they are the masses of equal volumes of substances, then the substance with mass 0.8 is denser. On the other hand, if the numbers refer to the volumes of equal masses of two substances, then the substance with volume 0.7 is denser.
What is quaintly known in the UK as Knock for knock. Equal blame, equal responsibility.
The earth and the rock both pull each other, with equal force, because of the way gravity works. We don't really know how, but we do know that any two masses always attract each other with equal forces that depend on the product of the two masses and the distance between them.
Tectonic plates can be formed when two land masses collide over time (also how mountains can be formed). In this case however one land mass slides over the other. This can be very unstable though which is why earthquakes happen.
Weather systems are formed when air masses of different temperatures slide over one another and collide with each other. The rotation of the earth, solar wind, and other factors are also responsible for the formation of weather systems.
equal and opposite depending on the masses
If their velocities are equal, then the speed of each bus relative to the other is zero. A passenger on one bus could read a newspaper held up by a passenger on the other one.