Good morning - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I see that your question hasn't received attention yet and I'm quite sorry. I think part of the reason is that it's going to be a difficult question to fully answer over the internet. Our job performance as experts is measured by our customer ratings, so sometimes when experts think they might get a bad rating, they'll shy away from a question. If you don't need any info anymore, please just disregard this posting.
If her teeth are as terrible as you're suggesting, I would definitely make some moves to get this fixed. Dental disease can be very very painful and it absolutely can stop them from eating. I've seen this in my own dog, who became a monumentally picky eater. Even though I didn't think there was anything painful going on in his mouth, when I cleaned his teeth, his appetite improved and he is now back to normal. Even if a pet is geriatric, unless they have a bad heart of some other major systemic disease that would interfere with anesthesia, I still recommend cleaning teeth as a quality of life booster. I would rather take the small risk of anesthesia than allow them to continue suffering with the dental pain.
Now constipation can be a different set of problems, and so can severe abdominal distention (I'd call it severe if she's truly as round as a honeydew melon). You could certainly try adding a teaspoon of canned pumpkin to her food, but if it's as severe as you're suggesting, it's probably going to take more than that.
Again, I'm sorry for the delayed response - just disregard this message if you're no longer in need of help. You can reply with more follow up questions!