Jay has training in contemporary dance and ballet and went from his course to being in the Wanted afaik. Anyone who has had drama training will have had dance training - my DS is a trained actor - he has had fairly extensive dance training, and in singing - but much more in dance. All actors have to be able to move properly. Also actors have training in - well - acting and performing - so big advantage.
The only unacceptable training IMO is if someone has had specific training in ballroom dancing, because that is what the competition is - learning how to ballroom dance. And now they have all had 13+ weeks of one to one intensive dance training anyway.
James Jordan is beyond bitter - he never won strictly because no one liked him, even though he had some pretty decent partners. I would never take any heed of anything he says - every time I've seen him on anything, Strictly, included he comes over as a thoroughly unpleasant little man.
Kevin openly admits playing to Len - that's not a crime - it's common sense, these are competitive dancers - obviously they will have an eye on the judges, the option of putting in stuff to catch the judges eye is open to everyone. The public who have 50% of the vote probably don't care though so others may take the option of going for style over substance to please the public - you takes your choice. Obviously if you can do both then you're onto a winner.