...so how do I explain to my security auditor, "I know it's not the
latest version but it's not vulnerable because a third party
supposedly fixed the vulnerability in a maintenance release that is
dated prior to when the vulnerability was originally reported." ?
That's about right, as far as the explanation goes. If it doesn't make sense to you or security auditors, I am sorry, but that is the way most Linux distros (Debian, *buntus, RHEL, CentOS) have been working for years. I rather doubt it's going to change soon, so, may be Ubuntu is just not for you, and you should look at something like Arch Linux, Debian unstable, or other OSs.
PS: You can check what's been fixed in a package in Ubuntu with the following:
apt-get changelog pkgname
For example, apt-get changelog openssh-server looks like this:
openssh (1:6.6p1-2ubuntu2.3) trusty-security; urgency=medium
SECURITY REGRESSION: random auth failures because of uninitialized
struct field (LP: #1485719)
- debian/patches/CVE-2015-5600-2.patch:
-- Marc Deslauriers Mon, 17 Aug 2015
21:52:52 -0 400
openssh (1:6.6p1-2ubuntu2.2) trusty-security; urgency=medium
- SECURITY UPDATE: possible user impersonation via PAM support
- debian/patches/pam-security-1.patch: don't resend username to PAM in
monitor.c, monitor_wrap.c.
- CVE number pending * SECURITY UPDATE: use-after-free in PAM support
- debian/patches/pam-security-2.patch: fix use after free in monitor.c.
- CVE number pending
...