So sorry about Spot. They can and often do go suddenly after seeming just fine shortly before. And unfortunately the only way to try and find out what happened is to do an autopsy and even then it may not be conclusive. You tried to get him to the vet. I’m sorry he couldn’t hang on. I know how hard it is losing your precious little rabbit. My heart goes out to you.
You are doing the right thing by giving Cookie extra attention and extra love. Monitor his pooping, eating, and drinking to make sure that all remains consistent. His behavior may change somewhat do to Spot being gone and it may not. Everyone rabbit reacts differently. As to getting Cookie another companion, I haven’t had experience with bonded bunnies after a death of one of them. There a many members here who have. It may take some time for people to respond to your questions. If you feel you haven’t gotten adequate feedback regarding your questions after a few days, you may want to start another thread in the House Rabbit Q & A forum.
Again so sorry about little Spot. Thinking of you and Cookie.
Binky free {{{Spot}}}