Hello. I'm sorry your post sat here so long w/o acknowledgment. I'm afraid I can't answer the 'why' of how your boy got so ill. Please know I send my deepest condolences.
Have you tried googling to see if any of your kitty's issues are common for a bengal/tabby mix? That's where I would start. Above all, I hope you're not blaming yourself.
We had to put Jade down a yr and a half ago due to kidney failure. She, too, had many issues, with the last being kidney failure...which is usually how it goes. Again, I am so sorry for your loss; I feel your pain. Hugs.
Another little life lost way too soon, I'm so sorry and I wish I could comfort you with some answers to why this should have happened.
They can't tell us when something's wrong and cats can mask symptoms so well, you had no way of knowing, the suddenness leaves you in shock along with the grief
I'm so sorry for your loss, RIP little one xoxox
How you doin'?
In rereading your post, it sounds like all may of been due to the stomach mass. As hard and as sad as it is, please be assured you did the right thing.