Without being able to review all of his test results and examine him in person, it's hard for me to point toward an underlying disease process. Based on your description, it sounds like he has a heart condition that may or may not be the cause of the edema in his leg. Edema, if severe, can cause pain and result in the limp you describe. Other potential causes of pain in the back and hind limbs include disc herniation in the spine, infectious or inflammatory disease of the spine, and spinal neoplasia.
My concern is that, in addition to his previous conditions including pneumonia, heart enlargement, and joint pain, he now is showing new signs including reluctance to walk and possible back pain. In my experience with dogs of his age, accumulation of issues that don't respond well to treatment or respond initially but then worsen or grow usually signifies that something bigger, more serious is going on. At this point we as pet owners are faced with a conundrum - how far do we go before we decide we've done enough? You did every test your vet recommended, and yet his condition is worsening. If you did 10 more tests, would it change the ultimate outcome? It is the hardest decision we ever make and yet we all have to make it at some point if we are fortunate enough to own a dog into their senior years.
If you wish to pursue additional testing, your vet may recommend things such as advanced imaging (CT scan or MRI) to locate the source of Snoopy's pain. They might also recommend referral to a cardiologist to further characterize his heart condition.
If you are feeling that additional testing isn't an option financially or that it wouldn't be the right choice for you or Snoopy for any reason, then we can discuss quality of life and end of life decision making. Snoopy's vet is also qualified to have this discussion with you (perhaps more than I am) because they know both you and Snoopy and your long term goals for him.
Either way, I am here for you and supportive of the path that you choose for you and Snoopy.