I am not a medic but was married to a DH with cardiac issues for a long time. The way beta blockers were described to us was this: when the body needs extra circulation, it sends a message to the heart to ask it to work harder. Beta blockers interrupt that message and slow down the heart's response - this is to protect the heart from over-exertion that sends it into irregular rhythms. They are not the only option, the right dose is important, and sometimes you might need a combination of other things too.
While you're adjusting to the beta blocker, it can feel awful, because those slowed down messages can make you feel exhausted and really unwell.
From what you describe, your DH's heart has been over-working while in the irregular rhythms, and now you also know about the valve issues etc. All those things could be causing breathlessness that is cardiac-related, so I would guess they are concentrating on treating the heart issues. If he is still short of breath after they get his treatment right for the heart, then they should explore whether there is an asthma issue.
A final word on ablation - essentially the burning away of the route that the electrical current is mis-directing down, so that the electrical impulse goes down the right path instead for a regular rhythm. There are lots of things that make someone a good candidate or not; my DH was not, because his heart was so damaged and enlarged that whenever they tried an ablation, the irregular electrical impulse just found another "wrong" route to go. He was young, so there's no hard and fast correlation to the age of the patient.
I hope that the hospital takes this opportunity to get his medication right under close supervision, and that he soon feels an improvement. Regardless, if you can find a friendly nurse, do ask about a nurse specialist for heart failure (horrible term, it doesn't mean the end is nigh, just that it's not working perfectly). They would be able to take the time to go through all the diagnoses and options with you, and answer your questions. Ideally the doctor would have done this, but he doesn't sound like he does "people" unfortunately.