9.5 year old cat passed suddenly

I have been wracking my brain since Wednesday when my wife and I came from work and found our baby, Nova, on the floor. He was indoor cat only and always had tons of energy. He was a loving cat, all of our friends and family adored him. We have been a complete mess to say the least, he was the first baby we saved together 9.5 years ago. We haven't had any answers until I decided to look online to try and put my mind at rest. He he was fine when we left for work that day around 7. We got home around 4:30 and he was laying there sprawled out, similar to the other posts I've read. We called around trying to find somewhere that could do a necropsy, I needed to know what happened. We have another cat and two dogs and our first thought was the dogs did it. The dogs have never gone after either cat but one did have a couple of scratches on her face and there were a few tufts of fur around him. I didn't want to believe the dogs could do this, they all got along quite well. We examined his body and there were no puncture marks or blood, he did have a few marks on his rear area. He also looked to have been licked all over.  No one could do a necropsy because of how long he had been gone (4-6 hours before we got home). We were able to get an X-ray that showed no broken bones or air pockets in his lungs. They said the marks on his rear didn't break skin and were either already there (like a scar) or had been after he passed. Nothing made sense. I have been neglecting my dogs the last few days, can't even look at them, thinking they were the cause. I also looked online and found some dogs will paw, nip at, and lick animals to wake them. I have read read so many posts about these sudden deaths, each one so heartbreaking to hear. I wouldn't wish this pain on anyone and my hearts breaks for all of you who shared your stories. I do thank you for sharing because now I have a little comfort knowing cardiomyopathy or other heart condition may have been the cause and that my dogs may have been trying to check on him or revive him. Now knowing is so hard. I wish there was something I could do. I wish I could have been there to hold him. 

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