Can this happen?
Yeah. Some dumbasses can get themselves hooked on heroin. Some genes are more prone to addiction too. If you’re extremely addictive in nature be extra careful.
Is marijuana some crucial link and “gateway drug” in this alleged chain?
No. Of course not. We’ve seen plenty since this 1960s propaganda offensive.
Don’t do heroin. Don’t do meth. Learn about these substances, what they do, and why they hurt you.
Don’t tell lies either. Or your credibility will evaporate.