I am so sorry about your dear little Bentley. Bless his Soul.
I don't know everything about heart failure, but from what I have heard, there are usually symptoms which last a while, and you would have noticed maybe at least for a few weeks?
It makes me wonder if Bently actually had a cardiac arrest.
With a heart attack, there can be some slight symptoms just before -maybe a day or two before. In many humans, those signs can often be ignored or blamed on something else, like indigestion or a strained arm, etc.
The thing is though, that a heart attack when it comes on for sure, is usually quite sudden. At that point, it's a fight for life, and not always a battle that can be won, even with the best speedy medical care.
It's hard to say if that 50 minutes between hearing the vet was closed and getting him to the emergency vet woul have made any difference to the outcome.
You did your best for Bentley, that is obvious.
There's an element of shock for you also, as you didn't have time to prepare for his passing.
I am so very sorry, Carole. My deepest condolences and kind thoughts.
If you find it difficult to deal with your grieving, there are online communities where you may find comfort and understanding with others who have lost beloved animal companions.