The IRS NCS will normally wear the inside of the tires faster than the outside, but depending on tire size, it may not be that much of a difference. The 15" ones will be the longest lasting, and they get worse from there.
If you do not rotate the tires as specified, the fronts will wear much faster.
There is a treadwear rating on the tire, under normal circumstances (tires kept rotated, inflated, etc.) that will usually correspond to the mileage within a margin. So if the rating is 500, then one could expect them to last up to 50k miles, but not always.
Swiped from one of our tire suppliers:
UTQG Treadwear grades are based on actual road use where the test tire is run in a convoy on a 400-mile test loop in Texas (West Texas, actually) for a total of 7,200 miles. The test vehicle can have its alignment set, air pressure checked, and tires rotated every 800 miles. At the end of the 7,200 mile test, the wear on the tires is measured and compared to a reference tire that was being run under the same conditions. If the test tire is expected to last as long as the reference tire, it receives a UTQG Treadwear grade of 100. If it is expected to last twice as long, it would receive a grade of 200. 300 means it is expected to last three times as long, and so on.
The reason the Treadwear grade may not be incredibly reliable is twofold. First, since the tires are only run for 7,200 miles, the tire manufacturers have to extrapolate the remainder of the data, and that can be open to some interpretation. Second, the tire manufacturers are allowed to under-report the Treadwear grade, just not over-report it. So if a tire technically may be able to achieve a 700 rating, the manufacturer (primarily the marketing department) might want to report it as 400 to make it "fit" better in a certain market segment. As a result, it is generally only somewhat helpful to compare Treadwear grades on tires from the same manufacturer, and we don’t recommend comparing grades between different brands
So, in short, a lower number Michelin will wear out faster than one that is a higher number, if everything else is the same. But you may get more life from a 600 rated Michelin than you would a 600 rated Bridgestone. Also beware that super high treadwear numbers mean typically a super hard tire, which may last a long time, but it will be worse at literally everything else a tire is expected to do: grip the road, be quiet, stay round.
VAG low profile tires wrapped around wide deep offset rims means the rims get bent... a LOT. Very easily. If you have a lot of potholes in your area, or routinely get romantic with curbs, you'll experience bent wheels. There is not a week that goes by that I don't see a bent wheel. Often more than one, on the same car... this week it is ALL FOUR on the same car. This will cause all sorts of weirdness, and the rims can be bent so bad that the rolling compensation the alignment machine's heads do cannot ever be expected to be right, so the alignment may be out of whack too but unless the wheels are true you could never know.
The NCS also tears through control arm compliance bushings fairly regularly. The NMS is worse even. While 50k would seem premature for this, I would still want to look closely at those. That will cause faster front tire inner edge wear, too. It is Wednesday, and I am about to do compliance bushing job number four today for this week. That's how common they are. Don't expect a tire/alignment chain store to properly check this stuff. They certainly don't around here.
Get a printout of the alignment. The NCS with IRS has a fully adjustable rear suspension, and unless you are in the salt wastes, it adjusts without too much trouble. And if the front is off beyond just the toe setting, the subframe can be shifted about a bit to get camber and caster back in spec (or at least closer to spec) if need be. In extreme (post crash) cases, I've also had to oval-out the upper strut mounted holes in the body, which can afford a bit more adjustment.