Very true, the two types of "spiciness" are quite different (wasabi is basically a ten-second flavor blast, whereas capsaicin sticks with you and accumulates). It's way more that just "mouth hot."

I remember one night, we ordered takeout sushi and got a side of edamame, too.

I wasn't paying attention and picked up what I thought was a bean that had fallen onto the sushi tray, and just ate it.

It was not a bean.

I don't think Japan spice levels are regional, it's just that actual spicy food is more widely available in cosmopolitan areas. Traditional Japanese food has basically zero spiciness outside of stuff like wasabi, which is a different kind of spicy that kicks through your nose and is gone in moments, or Japanese pepper, which makes your lips tingle but doesn't affect your tongue very much. Their chilis that get used in the spice mixes called ichimi or shichimi are a very low level of spice, and up until the past 10ish years, the hottest stuff you could commonly find was tabasco.

Because of that, you often ran into the same sort of issue in all places without a reputation for spicy, meaning that 98% of the restaurants will make nothing spicy no matter how hot you ask for it, and the other 2% will melt your face when you order the max level.

For sure, Japanese restaurants often have the option to make it spicier (either when ordering or as a condiment you can add yourself). However I was surprised at how many people I knew had very low tolerance and would only order what I'd consider mild (or medium if they felt like challenging themselves).

Absolutely, true wasabi has a much more delicate flavor, but it still packs way more of a punch than, say, even the spiciest Japanese curry. I was floored by how spicy Korean food was when I first tried it, especially by comparison with Japanese cuisine. Generally speaking, I don't think Korean and Japanese food have much of an overlap, except maybe in ingredients used and the prevalence of rice- and noodle-based dishes.