Where are you getting these facts? The only thing I can agree with is that "natural" drugs aren't a healthier alternative to synthetic drugs.
Flashbacks are an unexplained phenomena. Nobody yet knows what triggers them, and the LSD is NOT accumulated in your spinal column.
This "4 times with LSD and you're clinically insane" is bullshit. I have friends who trip more often than every weekend. They are not insane. They might suffer from HPPD (hallucinogen persisting perception disorder), but they are not "insane."
Cannabis is not physically addictive. If you develop a tolerance and a psychological dependence and decide to quit, you will NOT experience physical withdrawal symptoms. Unlike heroin, an addict will undergo excruciating physical symptoms.