I do the same thing when I butcher a hog each fall.
Meat is meat, the principles are the same. Fat content, marbling, etc. matter just as much in pork. When I shop for pork I look for marbling as well.
Loin and tenderloin as in beef, they are are lean cuts. If you add this to a lean cut of beef, you need to add some fat.
Every time I trim pork belly, ribs, shoulder, etc. I separate out bits of fat and save them in the freezer. Any time I'm making sausage or ground, if it is lacking in fat, i just throw some of that fat in the mix.
Sure - I keep bacon trimmings in the freezer and grind with beef for burgers. Cube your meat and place in the freezer then grind it when it stiffens up. Keep your grinder head in the fridge while waiting for the meat and fat to firm up. If you're going to run it twice through different die put it back in the freezer before the next run. If you're going to season it do it after the first run.
Sure it will, but between the pork belly (uncured bacon) and pork loin you are going to have some expensive ground pork. I'd save both of those things for other uses and buy yourself some pork butt, or shoulder. A lot cheaper and less work as it already has about the right meat to fat ratio.