Oh, interesting! So I went down the rabbit hole explaining how it works for inserting date/time, and the behavior is actually different. 
I just checked (being still in the United States region according to my device) and see that you are indeed right! The “Add event” popup does ask for the time in 24 hour time, even though inserting date/time insert is now in 12 hour time.
Insert date/time is a core feature while “Add event” is part of the calendar-and-event plugin. These are very separate features. So let’s focus here on the “Add event” popup which appears to have a UX bug, which you’ve perfectly documented in the first post of this topic.
Improving the UI in the composer menu is a totally different conversation. If you want to start a new topic about that, feel free.