Hi irjO, nice video! It was well done and pretty comprehensive.
I have never read Lethbridge, but I was introduced to pendulums and Tarot cards by New Age "angel people" people back in the 90's. Luckily I didn't throw the baby out with the bathwater, as I've had to unlearn almost everything I was taught back then! I've used a pendulum for divination for a long time. I'll describe my process so as to add to the data here.
I have two pendulums, one copper and one rose quartz, like you have. I use the copper one mostly but both work well.
What I do before doing a session is I say out loud: "Any beings present here that are not of the light must leave, now (e.g. STS)" and I repeat it three times (of course I have to excuse my selfish STS self LOL).
Then I hold the pendulum and "ground" by simply being present in the moment and clearing my mind. When that's done I then ask something like this:
"If there are any beings of pure light and intention of STO persuasion who are willing and able to provide information that me or my higher self does not have for the questions I intend to ask (implied: while letting my higher self instead provide the answers I need if it has them), then I would be very grateful if you could, through my higher self through me through this pendulum, provide answers to my questions WHILE NOT LETTING MY OWN PERSONAL ANXIOUS STATE get in the way of my receiving the truth." I found adding that last little bit has made a HUGE difference in the quality of my answers, especially when asking health questions.
I then do a calibration, where I hold the pendulum a few inches above my hand like you said your mom does, then I ask it two questions: "Please show me your no: is my name [X]? (not my real name)." If it responds, it moves horizontally as it does for your "no" answers. Then I ask it: "Please show me your yes: is my name [Y]? (my real name)." If it responds, it moves vertically for my "yes" answers, as for you. (Sometimes I get no response, which tells me I need to do the session another time.)
I then go through with my questions, "yes" or "no" only. I do touch it between each answer to "reground" it as sometimes it takes too long to stop moving before the next question.
Often when I ask a question I won't get a "yes" or "no"; instead, the pendulum will simply stay straight up and down and will. not. move!! That tells me I need to move off of that subject or that I'm asking the wrong questions. That also happens when whatever source is giving answers decides it's had enough!!
Overall I've had excellent guidance from the pendulum, particularly with regards to health concerns. The one drawback with my method as compared to yours is, with your letter board you have the option to ask questions that your "source" can answer directly, giving information like the C's. With my "yes" or "no" method I'm stuck having to trust that I am asking the right questions, and there's no option for me to know what I don't know. My source can't tell me what I need to know before I can ask the right questions in other words. So I might need to get a board like yours!