First, the average life span of the breed is ~13 years old. So, she has done very well here.
Now I do just want to comment on what you said about not being able to put her to sleep because of muzzle concerns. We need to remember that at the end of the day if she gets to a point where she is suffering, we cannot just leave her suffering and in pain as that would be unkind for a companion. Therefore, if you did have to have her put down in the future, just speak to her vet about sedation. They can either given you tablet form to make her sleepy before she gets to the vet or give her a quick sedation injection into her backside before putting her to sleep (they then get sleepy over 10-15 minutes). Once those are on board, they can give the final injection with no stress and no muzzle. SO, if it came to that, we don't have to leave her suffering because of a fear of muzzles.
Anyway, in this situation, if she is losing weight then we do have some serious concerns here. Weight loss in elderly dogs can be due to oran issues (liver or kidney failure), metabolic diseases (ie diabetes), or quite worryingly, cancer. And the last one notoriously steals weight from them while causing little other issue. Finally, I would just note that if she is struggling to eat that will also be thwarting her keeping weight on.
Now in regards ***** ***** actual question, we'd want to use a calorie rich food here. This way we can get more into her per bite to help combat that weight loss for her. There are canned options (ie Royal Canin Recovery diet, Hills' A/D) we often use in these cases. They can be fed directly but are made to be easily watered down for her to lap or you to syringe feed. As well, the same can be done with canned puppy food since it too has more fat/protein per bite than adult foods will. So, both are very much options to try here.
Otherwise, if a gruel of one of these is still to much for her, consider a liquid diet. The ones we use most are Clinicare or Dogsure. And just to note, all of these are OTC at vets, some pet stores, but can also be bought online.
So, those are the 2 types of diets we'd want to use here. The pate style canned foods are made to be watered down for dogs that have issues eating. And the liquid diets are for those that can only drink or need syringing. And all have more nutrition per bite to help you fight her weight loss, even if you cannot get much into her. So, do try these for her.
Otherwise, we need to monitor her generally and if she gets to a point where she cannot go on, you will want to speak to that vet about sedation before euthanasia to keep her from suffering until she dies.
Please take care,