Features "missing" when importing to Calendar and/or in basic Calendar functionality

I am a fairly new NextCloud user and am trying to migrate a handful of calendars off Yahoo!

When I import (some of / all of?) these Calendars I see a warning: “the file contains objects incompatible with the selected calendar”

The import proceeds – apparently successfully – but then I note 2 issues among the calendar event entries. These are implemented features in Yahoo! Calendar and may be the source of the warning:

  1. event recurrence seems to work differently than in Yahoo!
    a. if I go to the “repeating” tab I see “This event’s repeating rule is not supported yet.”
    b. even so, the events are shown as “repeating” – how is this possible (given 1.a)?

1a. some related questions:
a. will I completely bork a recurring by clicking “reset repeating rule”?
1. YES - it seems to delete ALL these recurring events! [bad form!]
b. in NC, is there a way to delete a single event from a set of recurring events? (this seems to delete all events in the recurring series – a big limitation for me!)

  1. event reminder-by-email is not yet implemented
    a. this works in Yahoo! and I have many such email-remindered events
    b. even worse, such events are unfortunately imported without any reminders – e-mail reminder does not, for example give a “local” audio or pop-up alarm

Where can Calendar Devs best be contacted to offer (non-programming) assistance?