It sounds like you unfortunately have gotten yourself kinda stuck, but I very much sympathize. I too have an account dating back to iTools, and for a long time it was a major frustration that I was stuck with that original email address as unchangeable for the Apple ID, unlike newer accounts. However, some time in the last, I dunno 3-5 years maybe? I can't remember now the exact time I noticed, but after over a decade of requests and fading hope Apple actually did allow me to change the email address for that Apple ID, which I shifted to my own domain. So for anyone else who hasn't checked in a long time, worth noting situation might be marginally better now.

Re: "mac.com isn't doing email anymore", all the original mac.com email addresses still work fine. Apple has played around with various domains (mac.com/me.com/icloud.com) over their decades of bumbling with online services but they made them all interchangeable for older users, mails to the original @mac.com emails still go through. Even originally made aliases (they allowed 5 with iTools) still work. Not sure what your issue was on that one.

Finally yeah, ""security"" questions are one of those horrible legacy anti-patterns that I will cheer to see finally be dead and buried. If you try to answer them honestly probably anyone can learn it with a bit of online searching, if you go for more obscure stuff they're easy to forget defeating the purpose. It's really best just to treat them as extra passwords, use random alphanumeric values and keep them in your password manager same as the password. Apple has also fumbled around with recovery over the years, at one point you had options to have a manual recovery key you could save but I think that's dead and can't set it up after already forgetting. Maybe if you go in person to a store with physical ID and evidence, if you had payment associated with the account and have that credit card for example that might do it.

If you have nothing of value tied to the account though probably no reason not to just abandon it.