
Much of what you need to know has been said already here. I've not had to deal with hearts, although both my two here, Whippet (11) and Basset (10) have heart murmurs. Neither is on medication - yet. I would just say that none of us can realistically be watching our animals 24/7 - she might collapse and die during the night when you are all at home as much as when you are not at home.
'In their eyes'. I had a diagnosis with one, after I'd found a mass in his neck, of lymphoma. Given his age at the time, we elected not to give him any treatment, just to let him live his days. I spent the first months jumping at every odd behaviour until by around 6 months, after I'd gone into denial big time (they were wrong!) he suddenly collapsed. Didn't come for his evening meal with the others. That was the end for him. So no 'there in their eyes' because you'd never have known about his diagnosis from how he was until the final day.
However, with most of mine, I'd know well enough when they'd had enough and life was too much of a burden for them. And yes, they had 'the look'. I even had one who, after weeks of not being at all well, spent a final night in her crate, with a terrible discharge going on. I then had her on the sofa (on a pad) and kneeling in front of her talking to her, she gently licked my nose and I just knew she was asking me to let her go, and that it was ok to do that. Makes me get tearful just writing this.
My advice to you is to talk all of this through with your vet who should be willing to make sure you knew how to best handle your dog with this.