Did some tests and got some problems when creating event:
Start date and time are cleared when adding party or contact mechanism
If i define a type, add a party and a contact mechanism. Then i switch to another type, select a second party with a contact mechanism following type and switching to previous type then contact mechanism of second party is updated with this type. (i know it's weird to do such thing).
if i remove a party and his contact mechanisms, can't save, getting "The values of "Contact Mechanisms" are not valid
The start and date issue is related to #12020 (closed) We will need to fix that probably.
I can not reproduce the type issue but I can reproduce the domain issue. I'm not sure if it is related to the module code as I do not see anything wrong. I managed to remove it by removing first the contact mechanism and latter the party. With this order you will be able to remove them.