Great thread. I think you're doing a lot for Ellie that as an only dog, she is fulfilled. Heck, you are doing a TON more than I ever did for Miko when he was an only dog (but back then I was working and going to school, poor thing was home alone a lot!).
Miko is the type of dog that likes to be alone, he'll play with other dogs, if he wants. When I got Koji, Miko was very standoffish. He wanted nothing to do with Koji. Koji, OTOH, will be a mess w/o his brother. He loves him and hates when we leave him behind. Trust me, it's not the us, it's Miko he wants.
So, like everyone else said, I think it can depend on the dog's personality, but I also think it depends on how much you offset the "need" for a playmate. With Koji, he definitely would need one, but that's because I am away from the house for 11 hours.
If you really think Ellie needs "doggie" time, I would suggest daycare. There, they start to make friends and have preferences of which dogs to play with. Can't say its any cheaper than a second dog, but I'm just throwing it out.