Pumpkins are certainly an American addition. But I would be interested in knowing exactly how the split happened between guising in Scotland and extortion (trick or treating) in the US. I think you're probably on the right track with blaming it on the Victorians. They have a well deserved reputation for putting a stop to a lot of fun public activities (and also being rather... erm... interesting in private), so it very well could have been them. Though I can't say for sure because I didn't see anything that directly addressed the split and how it occurred. But there were plenty of references to pranks occurring in Scotland while guising, particularly to the miserly members of society, so at the very least it has softened over the years.