I literally e-mailed the lady from BelMar about a minute before I posted and she has already e-mailed me back. She was so nice and very honest. She combined the last two questions into one answer. Here is what she said.

What health testing do you do on your Danes?
I always prelim OFA at about one year and usually, but not always OFA at two. I also like to CERF and get a heart clearance though cardio myopathy is not something that can be cleared. In other words a dog can have a heart clearance and still get cardio down the road.

What is the longevity in your lines? Do your Danes (or your Dane's parents and grand parents, etc.) live past the average lifespan? Longevity is important to me, and I'd like to get a Dane from lines that have lived 10 years or more.
Have you had any problems, health- or temperment-wise, in your bloodlines?
Belle my oldest is eight and very bouncy, active and healthy, her mother is still alive but her father passed away at 7 years. There seems to be more longevity in the females than the males. Of her 10 children (one litter of eight and one of two), two girls bloated and died. I know that the second one was stress related and should have never happened, she was moved into another home with hostile dogs and so kept confined in the garage. had I known, I would have taken her back. The other, I do not have much info on as the family she went to was not very communicative after the placement. One boy died of osteosarcoma just shy of his fifth birthday and one girl has recently been diagnosed with the same, she is 6 1/2. Dogs are 40% more likely to get cancer than people which is one reason I provide a VPI insurance policy with my puppies. It has a cancer rider included. Another boy had a heart condition and was put down at six. Two owners have dropped off the face of the earth and I have no information on the puppies that went to them. I have two daughters, one 6 1/2 and one 4 1/2 and they are in excellent health. The third girl (4 1/2) living elsewhere, is also in great health. I know that that looks pretty grim.
Danes are known as a heartbreak breed. I have had Danes live to 13 years and others die as early as four. About 4 years ago I switched to raw feeding and am convinced that it is responsible for the health and vitality of my Danes at home, compared to their siblings, though I won't really know myself for years to come. With my last litter, I provided each prospective family with books on raw feeding to answer their questions about what I was doing and now all the families but one are also raw feeding.
I am also much more cautious about who I place a puppy with. I am not really interested in finding show homes that are going to shift dogs around and stress them. Though not all show homes do. It is important to me that the lines of communication be kept open so that I can answer questions and get information.
As for temperaments. I believe that you have to work hard to make a Dane go bad. Assuming that it has been properly raised and socialized you can absolutely trust a Dane to behave. I have worked with rescue for years and even mistreated Danes have such solid temperaments when treated correctly. I won't say that there are no bad Danes out there but I have never met one.
Well I tried to answer your questions as thoroughly as I could. Please let me know if you have any further questions.