Have my degree in space engineering (that's space, not aerospace; no airplane stuff in my learning) so I figured I should give this answer a go.
The NASA engineers plan this type of system thusly: After deciding on the high-level mission parameters, they make a list of several different landing systems. For MSL, this included the final design, aerobraking, airbags, and a host of other possibilities. As time goes on, they strip away the less feasible ones (usually there is a slot named "new tech" that allows for new technology that we haven't thought of; this is usually stripped off early as we require a few years to ensure new tech is TRL8). Missions like MSL are usually in the works for a decade or so before launch, which gives lots of time for refining the mission parameters. At some point, it was determined that the mass of MSL was going to be too large for purely aerobraking deployment. Additionally, they decided that the mission-critical systems were too delicate for airbags, which can seriously affect finely calibrated equipment (the heaviness also had to do with the airbags being scrapped). Needless to say, one-by-one the alternative landing systems were eventually all discarded as unfeasible, not conforming to mission requirements, impossible, too expensive, or too risky (everything has a risk analysis associated with it. If anything puts one toe over the determined "too risky" line, it's gone). As mentioned, many of these alternatives were eliminated because of copious amounts of simulations that showed they wouldn't work. And I can attest that there were many simulations. The first (and later, the last) thing that the engineers do when they are given a new set of parameters or a new mission concept is to model it to determine feasibility, budget restrictions, timelines, etc. And these simulations aren't restricted to NASA; everyone does them and reports interesting findings. As an example, I am a physicist now, not affiliated with NASA in the least, but for the upcoming OSIRIS-REx mission, I've already run hundreds of simulations concerning orbital patterns, scan angles, data upload rates, you name it. And I've reported those findings to NASA. For something as big as MSL, they had physicists and engineers from all over the globe running tests and feeding back relevant data. Then, the engineers at NASA can take that data and narrow down the list of usable systems.
As for how they calculate impact forces, etc. We have a pretty good idea of the atmospheric conditions, the gravity map, and other important features of Mars. The many hundreds of people doing hundreds of simulations that I mentioned cover everything. NASA uses a type of "Google Mars" to pick a number of potential landing sites and a number of potential systems and then releases that information. The simulations are then run for every system at every location using the specific characteristics of that location. For our final result, we knew the gravity map and the pressure density. Additionally, we can easily estimate thrust velocity, approach velocity, and a myriad of other parameters. Then whatever is left that we don't know, we approximate (not kidding, sometimes nothing describes reality better than blind guessing). Often, using simple Newtonian physics and some powerful computers, we can then simulate almost exactly what will happen. Of course, at some point we actually go out and test an analog model on Earth.
This all being said, there is some unknowns still. Very few things have been used on Mars before, so every time we send a new probe, we're testing a new system there. While MSL landed smoothly, not everything does. I direct attention to the Beagle II Mars lander (aptly nicknamed the Mars Polar Crasher), which impacted the surface without slowing down. However, in all cases, we make sure that the chosen landing system is the most suited one for the job.
