1. Private exercise of of investment options usually occurs within the context of brokerages, hedge funds, and companies exercising their own stocks, no? For the first two, they have to be large enough to have lots of clients, and the last one is limited to one stock. I'd see the brokerages/hedge funds as a form of federation so that it doesn't make sense to distinguish it from exchanges on the centralization/decentralization spectrum, and stuff internal to a company doesn't really occur on the open market in the same way so I'm not sure it's relevant at all.

2. Percentage of trades isn't the only metric: ease of trades is also relevant. I could find a Bitcoiner on Craigslist and give him cash at a coffee shop for coins. Doing this with most traditional financial instruments would be difficult if not simply illegal. Coins for cash at a coffee shop isn't probably the most common use case, but it's certainly easy and possible, and that's relevant.