Cube rendering glitch, black lines.

Hello community, Yes once again I am asking about those odd black lines, not because I am unable to understand the answers on the other question but rather because I believe the approach to fix those black rendering artifacts was wrong to begin with.

I posted a detailed "error analysis", if you want to call it like that, on the forums four days ago, but noone even dared to comment on it. I hope to have more luck here now.

Here is the thread with the error, it's pretty long and I attached screenshots to it for better understanding and I feel like such a long post on UA might be a little bit unfitting, so I'll just give you the link. I hope someone can help me on this.

If you need further information, please ask!

Thanks in advance.