Ubuntu – Two root accounts, what to do

I think it is a security issue, because that "Aside from the potential of my non-root user account being compromised" can be rather large.

But there are other increased risks beyond that. For example, you've now opened yourself up to a theoretical exploit which allows one to change permissions in the screen socket dir (/var/run/screen on my system, but sometimes /tmp is used). That exploit now has an path to getting root, which it might not otherwise.

sudo has other advantages, if you can train yourself to use it for each command rather than doing sudo su -. It logs actions (which, unless you're logging remotely, doesn't meaningfully increase security, but does give you a trail of what you've done). And it helps prevent accidents by requiring intentional escalation for each command, rather than switching to an entirely-privileged session.

If two accounts have the same user ID, then by definition they are the same account. It is possible, but not recommended, to have more than one line in /etc/passwd (or other user database) with the same user ID; they are the same user, with different ways to log in.

You were using an unsupported feature. The risk of using an unsupported feature is that it sometimes breaks when the system is upgraded.

Expiring password is usually bad for security, as it causes users to choose weaker passwords or write them down on a post-it note stuck to the monitor. The only security benefit to expiring passwords is to eventually lock out abandonned accounts. Since the abc account is presumably one that you use as part of some kind of regular task, don't expire it.

You should probably change this setup, as it's fragile. What to change it do depends on what you use the abc account for (who has the password, in what circumstances it's used, what's abc's shell, is it present on multiple machines, …).