Hello and welcome. I'm sorry I didn't see your post yesterday.
You've provided the best link I've seen for cooking individual items at the same time in a slow cooker. There are a few posts here in the forum about this, but since there's no good way to search for them, let's just discuss our experiences and use common sense (which is allowed on the forum, by the way, although seldom encouraged

).
I have separated items with heat-safe containers and with foil. Foil has an amazing array of uses in a slow cooker. I once used a lidded bowl to cook rice in the cooker while a stew was cooking. That kept the rice and it's water separate and allowed removal when the rice was done and the stew to continue cooking. I have made packets of veggies much like those cooked on a grill and set on top of other items as they cooked.
Time of cooking comes into play. Most items are cooked in a slow cooker until they "fall apart" (usually about 200F+). This may not be desireable for faster cooking items like steaks cooked medium, chicken that should be cooked to 170F, and rice and pasta. Separate containers helps in this regard.
In the example you gave, the potatoes might need the longest cooking, with the steak next (depending on the cut), and then the corn which might be done first. I often have the luxury of being around to watch the slow cooker during the cook. I'd guess most folks don't since that's why many use the slow cooker - set it and forget it.
Not sure how related this is, but I often use aluminum foil balls to keep items out of the liquid - like a roasting chicken that I want roasted more than stewed.
Well, hope this helps. You've raised a fascinating question. Keep us here posted on what you learn and try.
Red