I'm not overly sure on how to answer the second and third points of your question, however, I am about to start second year of dentistry, and a girl from the dental hygiene and therapy course is moving onto our course after her first year because she did so well in her exams. So yes, it is possible to get into dentistry via this route, although probably getting accepted midway through the hygiene/therapy course as opposed to after graduation is a bit rarer