That's a good article but you can find all that information here on this board and other boards too. It's been there for ages and many of us have retyped most of it over and over.
I don't want to pick on you, but it's hard to believe that you hadn't come across these tips somewhere in the process of getting all the way up and through trimix.
Around here our water is cold and we're a long way from most of the caves so...all my technical diving was in a dry suit from day one and I had to master valve shutdowns early in my tech training...which was long before I went through cave training. By the time I entered cave training, reaching valves was trivial especially in the relatively light underclothes I use in the Florida caves.
When I learned to do it, I wasn't on the internet much and hadn't found the answer. None of my instructors gave it to me either. My wife and I got in the water and spent lots of time practicing and figured it out in the process.
I'll tell you something else. Cold and fatigue can sneak up on you such that even though you can easily reach them, actually getting them turned can be hard...so I reach back and "fondle" my valves multiple times on every single dive just to make sure all is well.
Maybe your instructor could have been more help but given your background prior to the cave course, I don't think you can blame him. You're already a certified "technical diver"? Already an intro level cave diver"? I mean, you knew you were going to have to do valve shutdowns, right? You knew what equipment you would be using? It didn't occure to you to get in the water with that equipment and check things out before going cave diving? If I were you, I'd be more likely to be critical of your earlier instructors than your cave instructor. That's my take on it.