Yes, that suggestion by vectis makes sense. Storming the gates of hell might be fair enough, but storming heaven seems a bit radical for a parson. St Michael's Mount sounds more likely, and it looks the part too.

Yes, it does seem an odd sort of route. Maybe the idea is that they were thinking of crossing the Bristol Channel by boat and sneaking up on London from an unexpected direction, having maybe picked up a few Welsh mates on the way. Though in fact I think pretty well the whole idea of a mass Cornish rising on Trelawny's behalf was made up by Parson Hawker.