To say it clearly:
I’m using 12.1, and don’t have specific experience with 12.2 yet.
Difficulties to answer your question result from a few points in your 1st post:
Your point 5:
Question 1: did you get the SUSE installer to install on /dev/sda7 and /dev/sda8 without modifying pratitions?
Question 2: the SUSE installer usually installs a boot loader. Which one was it? Do you recall that?
Your point 7:
In the end you didn’t describe the kind of problems that you faced.
And/Or you didn’t describe the difference in behaviour after the installation of openSUSE.
Especially the latter would be important.
A further important question:
Did you boot in UEFI mode when starting from the openSUSE installer DVD?
Otherwise an UEFI installation of openSUSE couldn’t have been taken place.
And hopefully you did use the DVD instead of a live CD for installation.
At least under 12.1 the live CD wouldn’t boot in UEFI mode, as far as I remember.