I think number of unspendable bitcoin is more than this because there are also lots of addresses which contain bitcoins and nobody have access to it, like addresses of wallets whose password were lost, known addresses of satoshi etc. However nice listing...
Yup thats right, its mostly because during 2009-2010 since the price was so low many people burned a lot of bitcoins. And as you said there surely are a lot more addresses that we don't know about, considering this 2771 BTC is a very less estimation.