Just following up on all of this. I decided not to do the heavy metals testing / detox after looking into it further. They're not really evidence-based. (The exception is if you have mercury poisoning, which is a medical emergency and requires hospitalization.). I also stopped seeing a naturopath after I learned more. Much of what they do is not evidence-based. She wanted me to spend hundreds of dollars for tests that wouldn't have been useful.

I did follow up with the second neurologist who was fantastic. Very attentive, knowledgeable, and responsive. Did several MRIs including a brain scan, nerve conduction studies (2) and muscle tests (2). Thankfully everything came back normal, despite the twitching I was experiencing getting worse throughout 2022. I was ultimately diagnosed with Benign Fasciculation Syndrome - which gets worse with stress, insomnia, and anxiety. Managing those factors keeps the twitching down to a minimum. I also started hormone replacement therapy because in the middle of last year I also entered menopause, which can exacerbate insomnia and muscle twitching.

Regarding body vibrations when falling asleep or waking: I now believe this is most likely Sleep Paralysis, which gets worse with fatigue or chronic insomnia. Just two nights ago, I had an episode of full body vibrations around 5 am, which even included auditory hallucinations and flashing lights. It was wild. The last time I experienced this was in late 2021. I am not the only one in my family that gets these sensations, just drifting off to sleep or emerging from sleep. This is the best article I have found yet on the topic. Maybe it applies to some others here. (I hadn't slept well for a few nights, and the night I experienced this, was the first night I got a deep sleep and a full 8 hours - which, ironically, makes Sleep Paralysis more likely, due to REM deprivation from the previous nights!)
https://www.verywellhealth.com/symptoms-of-sleep-paralysis-3014781
I really feel for how much fear is involved in this process and I hope and pray that everyone here gets good news, and clear answers in their search.