Yeah. This is the first case where Kase's behavior is clearly harmful and unjustified.
I haven't looked at it from this perspective. This is, indeed, a good point. Really curious how will they play off it - in many manga, it's just day like every day, but in real life, something like this could turn into a really major, potentially life-ruining issue.
Even if it doesn't, I don't see Kase getting out of this unscathed, since it's someone Yamada respects and who she explicitly told her not to worry about.
but Yamada discounting Kase's fears and jealousy and not taking her concerns seriously is just as big of a problem
Not really. Because, hypothetically speaking, it doesn't matter how much he would want to get with her, he can't take away someone who doesn't want to be taken away. I guess it makes sense in the teen relationship context, but they've been together for a good while now and Yamada has never shown any intention to stray. At some point, the lack of trust Kase has shown, in itself, starts to become poison to the relationship.
Yes, it is rooted in Yamada's inherent lack of self-esteem.
If anything, it's all rooted in Kase's self-esteem issues, if she thinks that all it would take for Yamada to drop her is a pretty face flower boy. This girl literary uprooted her whole life within a few months and moved across the country for her not even a year ago.