real time number of transactions world wide
You would have to monitor the block chain yourself, like you say I think it would count as abuse if you poll blockchain.info every single second of every day. The block chain is just a database so if you have programming skills or are willing to pay someone with the skills it wouldn't be too difficult. 

There are probably tools out there already that could do this such as libbitcoin, bitcoinj, etc. I'm not sure. You could even modify the official bitcoind client to log every transaction it receives to a database/textfile/whatever.

FYI the listtransactions RPC call will only list your own transactions, not everybodys.