Re: "iMessages signin problem : "it wants me to sign in with a user ID email that I haven’t used in 8 years.
I’ve lost the password and it won’t let me change the password".
Apple support have some articles online (links below) that may help you troubleshoot:
What to do: If you can’t turn on or sign in to iMessage or FaceTime - Apple Support
or
If you can't send or receive messages on your iPhone or iPad - Apple Support
If needed: Messages Support (info. / support / contact)
Eg: There is a "Get Support - start now" link (online chat, phone, email) at the bottom of their website (link below)
Messages for iPhone, iPad, Apple Watch, and Mac - Official Apple Support