OP I'm so sorry for your loss but please don't feel guilty. It sounds to me as if there was something seriously wrong but they hadn't properly got to the bottom of it.
Corvental D is used quite widely - and long term. I'm pretty sure there are dogs that owe their lives to it. But I'm not sure about the side effects wearing off so quickly. It's a sustained release drug that is designed to stay in the system for hours. My dog is on it - she was initially like a dog on speed and would be a bit hyper even in the early evening - after taking it in the morning. But she seems to be settling with it. I would rather she was on that than the prednisolone. They didn't warn me about the potential long term effects and when I researched and mentioned it she said she might normally have been concerned were it not for my dog's age.

It was my pushing that got her onto inhaled meds as an alternative.
As for lungworm (I've gone into this with my own dog) there is actually more than one kind and the commonly touted preventative isn't 100%. My vet should have tested for lungworm and didnt do I got my own tests done through wormcount.com. It's only £10, it's quick and there is no to-ing and fro-ing to the vets.
That said, testing generally involves collecting poo samples over 3 days so in all you are looking at a few days.
I had thought about treating her with Panacur anyway. But you have to be careful. Dogs can have a reaction to the more intense lungworm dosage. Also if there are worms, the die off of the worms can actually kill the dog. There is an alternative slow method that may be more suitable in some cases.
Anyway I have had one dog die that I blame myself for so I feel your pain. But it does sound as if something catastrophic was going on that possibly couldn't be fixed anyway. Believe me - I have spent hours on the net and on forums researching my own dogs' symptoms - so many tales of dogs seeming to rally - then dying. Much like people! And from what I understand, dogs are quite good at compensating for/masking their symptoms. When it becomes apparent something is badly wrong, a condition can already be quite far advanced.
'Could have, should have, would have, if only' are wonderful terms to beat ourselves up with. The thing is you gave your dog a good life and you did your best.
Take care x