They're gonna be fine! The only reason to keep the stems is for easier hanging to dry/cure. Just keep them somewhere dry with good airflow and use them first.
Love a little extra umami with tomatoes and cheese. Big fan of a drizzle of shoyu or tamari too. It's why anchovies on pizza are so good!
If you want it to be! I think the additions would overpower Kewpie's subtleties but that wouldn't stop me from using it.
Hard to dispell racist people's belief that kimchi is "gross" because it's "buried in the ground". Before refrigeration it was packed into jars and set in a covered pit not dissimilar to a root cellar for the same reason: cooler temperatures in the summer, protection from frost in the winter.
I suppose the same could be said for sauerkraut to a lesser degree. I see more posts about eating sauerkraut than I do about making it. Sourdough too, though it's a much higher ratio of people showing off their own loaves.
Ohioan here, love this stuff. Sometimes I crave it. I just know it as "Midwestern Sushi".
Maybe I should not be enigmatic and just explain briefly: Real mirin is rice & koji allowed to break down into sugars inside a high-ABV spirit, usually shochu, which prevents microbial fermentation, resulting in a sweet liquid that is still fairly alcoholic. Commercial mirin is usually just syrup.
Mirin is definitely not vinegar. Happy to explain the process, having made it myself.