Can’t answer about treating a dog with cannabis. I’ve had dogs that liked to inhale the smoke, one that ate an entire quarter ounce, and others that sneeze and want nothing to do with it…
What I do wanna say… Get a second opinion on the amputation! I had a boxer with cardnosarcoma in his knee. He was 6 at the time, so a little younger, but a boxer is a 10yr avg. Vet said if we did nothing, maybe 2 months. If we removed the cancer, and didn’t amputate, there’s a chance some can be missed, and it metastasizes, and spreads, so maybe 6 months to a year, but he figured at least 2 years if we amputated the leg. That was one of the hardest decisions to make because of HUMAN feelings on the missing leg and perceived pain and adjustment for our pup. We opted to amputate, and the day after surgery I remember picking him up, expecting a big bloody bandage, and a very wobbly dog, and instead he had a few staples, some pain med patches, and more energy than he’d had in weeks because the painful knee was no more. The knee literally tripled in thickness in a matter of 4-6 weeks prior to this, and it was hard, not soft like tissue, which is what prompted the vet visit to begin with.
He went on to live 6 more years, with our other VERY active boxer, and we knew it was the right decision… Vet did leave a little bit of his upper leg bone in place to act as a bit of a protector for his internal organs, and warned us of weight gain and to be careful he doesnt injure the one good back leg he had, and I guess we got lucky with him… Ultimately lost him to cancer, but 12 is OLD for a boxer…Whats the expected age of Marley? That really factors in a bit. But I had to share the story of Milo because I saw truly how much it improved his life and gave him considerably more time with us… Its a tough position to be in, so I like people to know that there are success stories out there 
If the pup won’t inhale, I wouldn’t force it, you’ll make her more uncomfortable really and probably like it even less. My boy Nero would follow a pipe around the room, my boy Rocco would gladly sneeze in your face if you blew a hit at him, he did not like it at all… Do NOT blow it in your pups ear. Supposedly that has an effect, but it can also present the right conditions for ear infections, moist exhaled air, smoke which is contaminants, not a good idea…
btw… Rimadyl is SUPER hard on the liver. Generally not prescribed in younger dogs because of long term liver damage. Learned that with past pups who needed it. My current boy was on it short term after tearing his CCL / repair…
My thoughts are with you and I commend you for doing what you can for your furbaby… Wish I had more answers for ya 
And Rescues that keep in contact with their adoptees parents are awesome. My rescue place has kept in touch with me as well…
