But if relatively safe & harmless nicotine use (such as patches or e-cigarettes) is a gateway to the unsafe & harmful smoking, then that's an argument worth taking seriously.

I don't recall seeing this argument until now. On the concept of the "gateway" anything, it's often abused. If some subset of patch users - who had not smoked yet - go on to smoke, then technically I suppose the patch could be called a gateway to smoking. But by the same token, playing video games could be called a gateway to playing them to death. Becoming a priest could be called a gateway to abusing children. And so on. Technically, even visiting a tobacco shop with its wonderful aromas could be called a gateway to smoking, because some will want to try a cigar.

Rather than lean on the easily abused "gateway" argument, attempting a quantitative argument would be more genuinely informative.