Horrible experience for Apple Care- screen flickers when turned o

So we have been an Apple fan for past 15 years or so, but going through a horrible experience with my Mac. The retina screen had some spots so we took it back for Apple Care, they replaced the screen.

When we got it back, on startup, the screen becomes partially green or yellow for a fraction of seconds (photo below), we dropped the computer back again and just got a call saying that they think the problem is likely in the software or the hard drive or a loose connection someplace in the system , so essentially they don't know, and they would like to erase my hard drive to run some tests.

Whatever little I know about the computer, I am having problem understanding this... how can you mess up my software or hard drive when you are replacing my screen.

Not to mention, when we picked up the computer this dude who was carrying my computer banged it against the door.

I am fuming.... it was an expensive system to begin with, but what I am worried most is what's on that hard drive...

BTW the dude wanted my admin password so that he could check if things were all backed up.