There is progress.
After I have uncommented line 92, I did not get a result immediately, as I mentioned in my earlier post.
However, as you have just pointed out, I had to visit the page first to get it to trigger the creation of the extra content block. That worked and I then had "It Still Does Nothing" content block visible in the back end. Please note that I had already tried this before commenting out the line 92 - I just didn't do it after I had commented out that line. Sorry about that.
I then changed the title of the block from "It Still Does Nothing" to "blargh" and got an extra second block. The original block added previously wasn't removed from the back-end. So then I had "It Still Does Nothing" and "blargh". When I changed the title again, to "shitzooi", I now had a total of 3 extra content blocks: "It Still Does Nothing", "blargh" and "shitzooi" even though I had only this one line:
<?php the_block('shitzooi'); ?>
When I removed the comment from the line 92 of class.MCB.php file, all those three content blocks disappeared from the back end and I was back to square 1: no extra content blocks visible in the back-end at all, not even "shitzooi".
I am capable of writing some PHP code but I do not know how to debug this. If you give me some instructions on what to try I'll give it a go. I now have a working solution for now at least, so I'm happy with that, but I'd still like it to be able to work 100%.