MISSION ACCOMPLISHED! Thanks everybody. OK, so let us finish the story for everybody's closure and potential future card twisters. So while waiting for the second replacement 7451, I decided to test all the remaining ICs. Also a friend of mine was able to look up that the 7451 does not behave like the 74LS51.
So almost the last chip I tested was the 74S175 at B1.
And it failed the test also but completely. So I get new hope I try to find a replacement. A quick look at the language card reveals it also has a 74S175. I decide to swap them and, BOOM, I get a nice "Apple ][" welcome after power on. So now all I need is another 74S175. Testing of the SD Adapter Card reveals that it is fried and will prevent booting if inserted. Thank you all for helping with my first chip level repair journey and all the helpful resources you provided.
I was on the wrong track because I realized so late that the 74LS51 apparently isn't broken but not the same as the 7451.
Kind regards and THANK YOU I learned a lot from all of you.