Yes!! There are three check balls total, two of them go down inside that opening of the shroud that goes around the drain impeller, so you did it correctly. One of them will push against the bottom hole of the filter assembly, preventing back flow, the other will cover the hole on the side of that outlet opening, which is where the remaining water is pulled from the sump. The third one is inside the little oval shaped cover that is secured with a T15 bolt on the bottom of the filter assembly. There's normally no reason to take it apart though. The purpose of that check ball is the prevent water being pulled from the filter accumulator chamber back into the wash impeller chamber when the drain portion starts, so that the soils that have been trapped don't sucked back out and through the wash arms. If you notice, during the drains, despite the impellers reversing, there's still a fair amount of water that gets pushed through the arms, just at a fraction of the pressure and volume.
You're exactly right about the build quality. Despite the tubs being plastic, it is GOOD plastic. There's a reason they gave the tubs 20-25 year warranties; there's no reason they'd ever crack or warp or leak. These machines are built like tanks, and you can feel it on everything from the motor and pump assembly to just opening and closing the door.
You'll be amazed what a power house it is at cleaning. The Normal cycle is beyond adequate for even the most heavily soiled loads. My aunt has a 2003 model similar to the one in your original post on this thread, with the difference of one addtional cycle I believe. That particular machine is what made me fall in love with the PowerClean design and Whirlpool in general. I was always amazed at how that machine was packed to the brim and not a single dish was rinsed, the door covered in goopy sauces and even pieces of green bean or a stray noodle or bits of cooked hamburger meat sitting on the bottom of the tub. A normal cycle later and not a single speck remained and nothing but a fresh smell at that. I envied her for that machine because we had a GE Nautilus that would leave yiblets and grit along the top edge of the door even after a Pots and Pans cycle, and still didn't leave the squeaky clean feel that the PowerClean did on just a normal wash.
Even for the machine I have, I use the Normal-Low Energy cycle more than Normal itself, because Low Energy forgoes the prewash and does a longer main wash, better for enzyme based detergent pacs today, and still heats the final rinse to 140. Not a single complaint with any of the results.