Pictish crossbows in the 3rd-5th Century?!?!? Do you have a link or pics I'd love to see that. As far as the Roman crossbow is concerned, it appears to be an indigenous design. As far as I know there's no evidence for the spread of the crossbow from the Han to any of the people living between them and Rome. Also, if I remember correctly, Roman and Han crossbows were designed very differently. HowStuffWorks "How Crossbows Work"
The first page of this article has a diagram illustrating the difference between the Han and Roman trigger mechanisms.