Since I got no solution to the above I kept researching and found the explanation to prevent people from going through my ordeal:
If you are up to date with Apples security updates you won't be able to install the 10.10.5 Combo because of below:
David Empson posted on MacInTouch on 1.7.16
I've just answered a similar question on Macintouch, in that case with
reference to the 10.8.5 combo update not being installable any more.
Checking the details for 10.10.5...
Yep, same answer.
1. The installer script for 10.10.5 combo requires the target system be
running an OS X version 10.10.something (less than 10.10 or greater than
or equal to 10.11 is excluded). You are on 10.10.5, so OK so far.
2. The installer script also requires the target system be running build
14A388a or later, and build 14F27 or earlier.
10.10.0 general release was 14A389, so the 14A388a cutoff is presumably
the final public beta or developer gold master - anything earlier would
need a full install of 10.10.0 or later to be able to update to 10.10.5.
10.10.5 general release was 14F27. The combo update will not install if
the system build is greater than 14F27.
The latest security update for Yosemite (2015-006) sets the system build
to 14F1509, which is greater than 14F27.
At some point, a post-10.10.5 security update has incremented the system
build number, which prevents the 10.10.5 combo update from being
The most likely reason for this is that the changes in the security
update meant reinstalling the combo update would leave the system in a
broken state, so Apple deliberately increased the build number to
prevent the user breaking their system.
The same rule applies recursively for subsequent security updates,
preventing reinstall of earlier ones (once over the critical point where
the combo update isn't safe, Apple might as well keep incrementing the
build number with each security update, making it easy to tell which
update was the last one installed).
If you really want to reinstall 10.10.5, you can still do so using the
full Yosemite installer (which will set up new System and related
folders in the process, so incompatible changes from later security
updates are not relevant). --