Is this canning method safe to use for spaghetti sauce? (Boiling the sauce in the jars in the oven)
No, it is not safe. You need a pressure canner. That's what the USDA says about anything containing meat:


  There are no safe options for canning these foods listed below in a boiling water canner.


See http://nchfp.uga.edu/how/can_04/soups.html for a table of pressures and processing times.