I've continued to look into this. Here's what I've found so far:
I have special email addresses for my logins, and this is not the current email address used for logging in at Kobo. However, it is still the address I get the Kobo email newsletter from.
So this isn't related to a current login.
I rarely use the same email address for different accounts these days. I signed up to the places mentioned in my first message back in 2012, so that's a long time ago and before I got strict about how I use email for logins.
For years now, I've been using unique, self-hosted email addresses when I need to set up an important account. It's a simple way for me to add extra security to my accounts. If the email address is also unique, data leaks and hacking won't cause so many headaches for me. I don't have to run around changing my passwords at a bunch of places because one place got lazy or hacked (or sold off my data!). And if my email address gets sold, I know it. I mean, if I get an email to something like fasttaxes412@(mydomain).com, and it isn't from fasttaxes (not a real place), well, we know what that means. ;D Woe to my heirs though when something finally happens to me because I have a lot of unique email addresses. I've saved the info for them and hope they can muddle through... Most forward to my main email addresses so it's not as much of a hassle as you might think.
I have found one other distributor where I used this email address, but it was not for login. It was for their newsletter. So that brings the total to 3 accounts and two newsletters.
I'm going to go back and strip out Kobo from the original (just because it reads a bit like an accusation up there), since I now think it's one of two possibilities. I don't want to tarnish any reputations until I'm convinced they deserve it. And I can't be convinced if there is another possibility, even if it seems remote at this point. But I'm still annoyed, and I'm probably going to keep looking into this until I feel confident I know how this "Elite Authors" business got hold of both this email address and my legal name.
Honestly, I wouldn't care if it weren't for the legal name part. I realize stuff like this happens all the time and data is for selling and woe to anyone who is naive enough to think it won't be sold or gathered and collated in some way. But I have such a unique legal name that I don't like it being linked up with my pen name publicly in even a round about way. My personal info like address and phone number is too easy to find online once someone knows even my first name (it has a unique spelling, despite being so, so ordinary when spoken aloud). That's the hassle of having a legal name that is shared by exactly 0 other people in the entire world as far as I can tell. :o