Cancer or infection?

I recently had to put my dog to sleep after I found out he had cancer that had spread to his lungs. I am wondering if more could have been done if this was discovered sooner. In late Sept 2013, I brought him into the vet after noticing his glands on his throat (bilateral mandibular lymph nodes) were enlarged. The vet recommended giving antibiotics (clindamycin) to see if that would reduce the swelling, thinking it was related to his teeth and plaque build up. The antibiotics didn't work reduce the swelling, but the vet recommended to continue antibiotic treatment the first five days of the month in what she called pulse antibiotic treatment for periodontal disease. In late November, I noticed my dog was coughing more than usual, but thought it may be kennel cough as he went to doggie daycare once a week. After it didn't clear up with the antibiotic treatment (the first 5 days of Dec), I brought him in and he was diagnosed with kennel cough and given enrofloxacin and tussigon. At that point he had lost 2 lbs and his lungs sounded harsh and slightly congested. The night we got home from the vet, he started not eating his full meal and his appetite was decreasing. After it didn't get better, I brought him back in six days later and a chest x-ray was done and spots were found on his lungs. He was diagnosed with metastatic lung disease and lymphadenopathy. Two days later I put him to sleep as he was not eating (even treats) and was coughing quite a bit and restless at night.

So here's my question...should more have been done back in Sept when I discovered his glands were enlarged? Could anything have helped at that point? I'm frustrated because I feel like by the time we figured out what was really going on, it was too late for treatment. I was also frustrated because the vet who took the x-ray indicated that the cancer was highly treatable, but the vet I consulted with the next day indicated there was likely not much that could be done and my dog had days to maybe a week to live. I am considering adopting a dog, and have doubts about using the same vet. Am I wrong to have doubts about the treatment my dog received?

Sorry about the crazy long question, I wanted to make sure to include all the information

Thanks!
Laura