I am traumogenic (trauma created/clinical DID) plural system. It is highly likely that I never had a "core personality" that I was born as and instead have always had either an infant mental state or multiple personalities. (During the time most babies begin to form a coherent personality, I formed several. It is possible that not every system forms like this but highly like I did)
Every time I've felt I was singular, I've experienced mental walls, amnesia and other DID symptoms. If someone could do a "I'm singular ama" that'd be cool because I have questions too. :)
Depends very much on the alter and every system is different. Some alters can get very involved. Others might want to front for a little bit and then stop without wanting to be the host. There is a pretty active alter who I adore that while vocal, I don't think she really wants the hosting jobs I hold. Some alters are currently entangled in trauma loops and still think the past is happening. They need help/rescue. Others only communicate a little bit. Many Minds on the Issue podcast has a few episodes on ways to get at least a small working team going and that if even only one alter wants to have a collective system then that alter should persist and keep trying.