I have some experience with CHF. My Boston was diagnosed with it and lived approx 1.5 years after his diagnosis. We put him on lasix and enalapril right after he was diagnosed. He did develop kidney failure which was what I ended up losing him to.
It's a very hard decision to make, whether or not to put him on he heart meds. I wasn't given a choice by my vet and didn't have any prior knowledge of the side effects at the time.
I'm not sure I would have put him through the treatments/medicines after seeing what he went through. He had to go on Pepcid because he wasn't interested in eating. Apparently ulcers are common in pets with kidney problems.
I eventually cooked his food. He refused dog food and it was very hard to find foods to stuff his 7 daily pills into. One day he'd eat hot dog and the next wouldn't touch it. Getting his pills down was a daily struggle.
I would cook 1 lb of small noodles, 1 or 1.5 lb of ground beef (or chicken breast), 1 bag of pureed pea/carrot mixture (look for bags that only contain the veggies - no salt), and some low-sodium broth. He really liked it and by that time it was more important that he WAS eating; it mattered less WHAT he was eating.
I also had to do subcutaneous fluids every other day on him for probably 6 months before I lost him.
Looking back, I'm not sure it was fair to put him through all of that. I know I was keeping him around for myself. He was my best friend though and I know now that I was selfish not to let him go sooner.
I wish you all the best of luck and hope that your Yorkie lives a healthy, full and happy life until his time comes. It sounds like he's in good hands.
I offer a few thoughts based on my experience with human heart problems, mine. I do not have any experience with dog heart issues, but I bet there are a lot of similarities.
First the mitral valve problem may have been call Prolapse, or just leaky. That condition can cause heart enlargement, specifically in the atirum chamber above that valve (humans). It is not reversible unless the excess pressure it taken off, such as a mitral valve repair or replacement (me, my heart is no longer growing, but it did not shrink back after mitral valve surgery). I am not suggesting open heart surger for your dog.
High blood pressure can also cause general heart enlargement. Did you mention that? If not ask you vet about it. There are effective blood pressure medications for people, assume similar stuff exists for dogs, such as beta blocker and calcium channel blocker.
As will all of us aging the balance that seems best is to address Quality of Life. If the treatment is worse than the diastase, some medication side-effects can be very troubling, and the poor dog can't talk.
At the age of the dog I would discuss with the vet quality and extension of life issues and treatments, not seek cures. There is no cure for growing old. Well a healthy life-stile is a good underpinning, but that has to be done along the way, not after senior age is reached.
I hope I am not spinning a negative reply, I do not intend to be negative and I know about loving a dog, and people too, and know life and death decisions are very difficult and the "right" answer is not obvious.