Yes, that's your account ID. It doesn't only look like a Bitcoin address, it is one. BitWrk took a lot of inspiration from Bitcoin technology. If you want to, you can also import that key into your wallet: the WIF file is in /.bitwrk-client/privatekey.wif
Should I backup that file so that I could restore it if I reinstall on the same computer or on another?
Can I have the same ID on multiple computers at the same time or should they stay unique per computer?
I see the 20 mBTC on the account page.
But it's not on the blockchain:
https://blockchain.info/address/18P5aa4tfPbSuPeVpdAvHossQkWrZ6TK2g