Can headmates come and go? I feel like I might’ve had one or two other headmates than i do now when I was younger but I’m pretty sure they’re gone now.

Headmates absolutely can come and go. They can be there, or gone for days, weeks, months, or years. I have had headmates who only appeared a few times a year. I have also had headmates who were around all the time for years, and then gone for years, only to show up again.