Thanks so much, AL!
Excellent compendium of information, I am making a large document with all your images and comments, thanks! Thank you also for noticing the missing character in the {lcr} I used for the two equation example with {array}.
In my first try with the broken matrices I failed in all but one..., I am not sure about the reason yet. I had noticed that when copying and pasting a syntax that works, I can slowly modify (one character at a time) an expression and get with patience the rendering I want. I am happy I had one that rendered correctly, and used it as my template. The process was tedious but I had some success with that. No matter how identical the expressions are, some render correctly and what I would consider a clone does not.
What I started doing in the last few hours was to copy and paste expressions that seem to work not in the Moodle editor, but directly in the HTML source. Since then, I have made much quicker progress, because I can edit the full pasted expression in one go instead of in small steps, and get it to render correctly.
Although at this point I consider magic, or just luck to be able to have an expression of this type working as it should, I have the impression that the Moodle 2 editor could be introducing some non-printable characters "invisible" to the user that are precluding the rendering of a possible corrupted TeX syntax... It may sound crazy or just that I am very brain dead at this point . Anyhow my conclusion is that it is much more efficient to enter these somehow more complicated math expressions while viewing the HTML source and not via the standard editor.
Thank you again AL for sharing with all your enormous knowledge of TeX syntax. I am forever grateful!
Cristina