Hi there, Ewan! thanks for getting in touch and welcome back to the forums. π
Good questions. Let’s see if we can tackle them one at a time.
is there any way of changing the look of the mini calendar for events that are on for a long time?
Unfortunately, there is no specific CSS in the mini calendar that targets multi-day events. You can target specific individual events, but not a type of event right out of the box.
Is there any way to mark the beginning and end of an event with dots leaving the ones in between clear?
As far as marking the start and end of an event with dots, you may need to customize the template itself (mini-calendar-widget.php) or here is a direct link to the documentation for the function that controls the day output in the mini calendar. This bit of customization is beyond what we can provide here in the forums, but this should at least get you started in the right direction for making that happen.
Sorry I’m unable to give you a concrete solutions, but does this help answer your questions? Please let me know. π
Cheers!
Geoff