Why does recent transactions tab on client show me these mined blocks?
I've just set this wallet up freshly, never mined nor sent any balance to this client, yet I can sign messages from these addresses
Your wallet is out of sync. Let it load the network blocks first (make sure you have network connectivity) and then start mining. But mining with the wallet isn't really good anyways, so you should disable it and mine with a GPU miner if you want to.