Hypotheticals that would offer a counter point to those saying those points show he is not guilty. So no, I don't consider him guilty or not, I don't know.
And I'll add this; someone would have had to have access to her keys, it's unlikely she would have removed all other keys, no other keys were found ever, the one key that was found is the so called valet key not the main one, so unlikely again that if she had only the car's key on her that she wouldn't have kept the main one instead. All this points to someone taking her key set, removing the others, keeping whichever works to drive the car, gets rid of the others just as all her other belongings.
Now somehow the cops would have had to get that key at some point, remove the other keys for no reason if the other keys were present, not plant it on their repeated searches, but instead hand it over to an independent investigator who would then plant that key. That's many layers of reaching. It becomes one heck of a conspiracy.
Is it surprising that they didn't find the key initially? Yes, but that depends on what kind of search took place, investigators will not always move everything, they will usually secure the location, and then it may stay as such for a while. I'm not familiar with the number and thoroughness of the searches they carried in that specific room.