All those symptoms you describe prior to a block are caused by anticipatory anxiety, which raises cortisol levels and adrenaline and dopamine in your brain. This affects us physiologically by exhibiting those symptoms you’ve described. Stutterers have a dysregulated dopaminergic system in their brain which mainly affects the speech motor centres. Singing relies on a different part of the brains timing system, or an external timing source rather than the internal timing source for fluent, natural speaking. Whispering doesn’t rely on as much muscles, vocal folds, air as natural speaking does at a higher volume. Yes, you can minimise stuttering by using fluency shaping techniques, but the moment you stop using those techniques, you will relapse. Our brains are simply wired differently to 99% of the population who don’t stutter.